What a subject line can and cannot do
A subject line has exactly one job: get the email opened. It cannot answer the question that stopped the purchase, it cannot make the shipping cost smaller, and it cannot reassure anyone that the size is right. Every one of those happens after the open, or not at all.
This matters because of where the money leaks. If your cart emails open at 40% and convert 10% of openers, doubling the subject line's performance is impossible, and improving it by a fifth moves the final number by a fifth of a fifth. Meanwhile the majority of the list never opened anything, and no subject line reaches them, because they are not reading.
So treat the subject line as a cheap, fast improvement worth making properly once, rather than the thing you optimise every week. Write forty, pick the ones that fit your store, test them in pairs, and then go and fix the email underneath them.
The four patterns that actually work
Almost every subject line that performs on a cart email is doing one of four things. Knowing which one you are using is more useful than a list, because it tells you what the email after it has to deliver.
- Specificity. Name the thing. “You left the oat linen set behind” outperforms “You left something behind” because the second one reads as a system notification and the first reads as a person who looked.
- The open question. “Not sure about the size?” works because it names the doubt before the shopper has to admit it. It only works if the email then answers that question, which is the part most stores skip.
- Service framing. “Did something go wrong at checkout?” assumes a problem on your side rather than indecision on theirs. It gets opened by the people who hit an actual error, who are the easiest recovery you will ever make.
- Honest scarcity. “Two left in your size” is fine when two are left. Fake deadlines work once, and then they teach the customer that your urgency is decoration.
Subject lines for the shopper who got distracted
Email one, about an hour after they leave. Most abandoned carts are not a rejection, they are an interruption, so this email only has to be a tap on the shoulder. Swap the braces for your merge tags.
- “You left the {product} behind”
- “We saved your cart”
- “Still thinking it over?”
- “Your {product} is waiting”
- “Picking up where you left off”
- “One step left”
- “Back to your cart”
- “{First name}, your cart is still here”
Subject lines for when checkout broke
These get opened by people who hit an actual error, which is the easiest recovery you will ever make. They are also the only group where assuming the fault is yours is both generous and usually correct.
- “Did something go wrong at checkout?”
- “Did the payment not go through?”
- “Quick question about your order”
- “Was it the checkout, or was it us?”
- “If the card was declined, try this”
- “Tell us what went wrong”
Subject lines for size, returns and shipping doubt
Email two, around 24 hours. This is where the money is, because it names the specific thing that stopped the purchase. Pick the group that matches your most common objection and delete the rest, since the objection is the part that does not transfer between stores.
- “Not sure about the size?”
- “About that size question”
- “How the {product} actually fits”
- “Most people size down in this one”
- “The measurements, in case that was the holdup”
- “What {product} looks like on a real person”
- “Free returns, in case that was the worry”
- “30 days to change your mind”
- “If it is wrong, we pay the postage”
- “Our returns policy, in one sentence”
- “Shipping is on us over {threshold}”
- “It can be with you by {day}”
- “About the delivery date”
- “You were {amount} from free shipping”
Subject lines that lean on proof or scarcity
Proof works on the undecided. Scarcity works once, and only when it is true, because a shopper can verify a deadline by waiting for it.
- “{Number} people bought this last week”
- “Why people keep buying the {product}”
- “What buyers say about the {product}”
- “Rated {rating} by people who kept it”
- “Two left in your size”
- “That thing you wanted is nearly gone”
- “Your cart expires tomorrow”
- “Back in stock, and going again”
Subject lines for the last email, where the offer lives
Around 72 hours, and the only place a discount belongs. Hold it back this far and it is a tiebreaker rather than a reflex.
- “Last call on your cart (and {discount} off)”
- “{Discount} off, until {day}”
- “We will meet you halfway”
- “Closing this one out”
- “Final reminder about your cart”
The subject lines that cost you money
Some lines open well and do damage anyway. These are the ones worth deleting from your swipe file.
- A discount in the first subject line. It costs margin on every shopper who was going to buy regardless, and it teaches your repeat customers that abandoning a cart produces a code. Some of them will start doing it deliberately, and you will have trained them yourself.
- Fake deadlines. “Expires in 1 hour” on a cart that does not expire is a claim your customer can check by waiting an hour.
- RE: and FWD: prefixes. They lift opens by impersonating a reply, and they cost you the trust the email needed to convert.
- All caps and stacked punctuation. Filters read them the way people do.
- “You forgot something!” Nobody forgot. They decided not to, and the line tells them you were not paying attention.
The sender name and preview text do half the work
The subject line is one of three things a shopper sees in the inbox, and it is not the first. The sender name is. An email from a person at your store beats one from a brand address, because the first suggests a reply is possible and the second suggests a system.
Preview text is the other half, and it is the most wasted space in e-commerce email. Left alone, most platforms fill it with the first line of your template, which is often “View this email in your browser”. Use it to extend the subject rather than repeat it: subject “Not sure about the size?”, preview “The measurements, and what most people pick.”
Test the three together. A subject line tested against a different sender name is not a test, it is two changes at once and no information.
How to test them without fooling yourself
Cart lists are small, which makes subject-line testing harder than it looks. If your winner is ahead by three opens out of two hundred, you have measured noise and are about to make a permanent decision based on it.
Test one variable, give it enough volume that the gap is obvious rather than arguable, and judge it on revenue per recipient rather than opens. A line that pulls in curious openers who do not buy has made your reporting look better and your month worse.
And keep the winner only as long as it wins. Cart subject lines fatigue faster than broadcast ones, because your best customers see them repeatedly.
What no subject line will fix
Here is the part the swipe files leave out. Every line above competes for the attention of people who open your email. The rest of your cart list, which on most stores is the larger half, never opens anything, and the reason is not your copy. Around 70% of online carts are abandoned before checkout (Baymard Institute), and email reaches only the fraction of those people who still read promotional mail from a store they have not bought from yet.
That gap is a channel problem, and rewriting the subject line a fifth time does not touch it. Stores that recover meaningfully more usually add a second channel rather than a fourth email: an SMS, or a phone call while the intent is still warm.
It is worth being concrete about why a call behaves differently. It does not need to be opened, it happens within minutes rather than hours, and the objection that stopped the purchase gets answered in the conversation instead of being guessed at in advance. A subject line has to predict the doubt. A call can just ask.
