The first product sets which message a customer needs next. Write it down once.
Your post-purchase flow sends every new customer the same mail. They didn't buy the same thing. A full-size routine buyer, a trial-size buyer and a gift buyer each need a different next message. Branch the flow on the first product and each of them gets the one that fits.
The repeat rates behind these branches, and what a returning customer costs through each, are in Cost per Returner. Map every SKU to one of the three before you write a message.
Full-size routine. These customers bought a habit. Send how to use it, timed from delivery rather than checkout, because a tip that lands before the box has nothing to explain. Then the next product in the same routine, then the refill on the measured reorder gap. Leave codes out of this branch, because a customer building a routine will reorder at full price, and a code teaches them to wait.
Some full-size buyers want the same item again. Measured: about 89% of repeat lipstick buyers in the skincare file rebought the same item. Where the second order repeats the first, lead with a one-tap reorder of that item and put the shade or size in the subject line. Judge that on two numbers, same-item reorders and cross-category orders, because they move separately and an average hides both.
Apparel works the same way. I wrote email and SMS for UNTUCKit (2015–17), a basics brand where the second order is a second color of the same shirt. Open rates ran above 40%, before Apple's Mail Privacy Protection made opens unreliable, while site conversion moved from under 1% to about 3.5% (reported).
The other two branches get their own sections below. Gift buyers leave replenishment. Trial and intro buyers need graduation, which is a different message from a refill reminder.
Flag gift orders when they're placed: a shipping name that differs from billing, a gift message, gift wrap, or a gift-only SKU. Take those buyers out of replenishment, because a reorder reminder for a product they never used teaches them to stop opening. Send one plain message instead, with no discount. If they don't answer, leave them alone until the next gifting season.
Subject: Was this a gift?
Body: Your last order may have been for someone else. Pick one and you'll only get mail that fits.
Buttons: [It was for me] [It was a gift]
Nobody needs a reminder to reorder a sample. The trial buyer needs a reason to move to the full size, sent while the sample is still in use. Take an invented 14-day sample, with days counted from delivery:
Run it on the back file first. Send it to the travel-size entrants you already have, less anyone you can't mail, and hold out a random half. On the skincare file's 489, a 5% graduation line would mean about 12 of the roughly 240 treated graduating against almost none held out, which is readable.
A 2% rate isn't, and that's an answer too: below your line, the trial stops being an acquisition product. Set the line before the flow goes out, because a line drawn after the result will bend to fit it.
On Shopify, Klaviyo records every line item as its own Ordered Product event, and Placed Order carries the item and collection lists. What it can't tell you is which product came first. A segment finds everyone who ever bought retinol, not the people who entered on it. So write the entry product to the profile once, on the first order.
The order can start the flow before the property lands, so don't branch at the trigger. Trigger on Placed Order with one flow filter: placed order zero times before this. Add a short delay (an hour is plenty), then branch with a conditional split on Entry Product Class, which Klaviyo reads when the customer reaches it.
Or have your middleware fire its own "Entry Product Classified" event after the write, and trigger on that. The no-developer path: Shopify Flow can tag a customer with an entry-product class when their first order is created, and Klaviyo's Shopify sync brings customer tags onto the profile. Confirm the property name in your account.
The rules for naming and guarding the property are in The Data Contract, at the back of the book.
This is one chapter of The Second Order, which is free and readable in full on a single page with no form in front of it.