Two of your storefront's opt-in surfaces are pulling in phone numbers right now. The popup that fires ten seconds after someone lands. And the checkout consent box that appears once they've already decided to buy. Both fill your SMS list. Neither fills it the same way.
The popup wins on raw volume — often by 3-4x. The checkout opt-in wins on downstream purchase intent, sometimes converting at twice the rate on your first few sends. If you only look at subscriber counts, you'll over-invest in the popup and quietly drag your cost-per-revenue up. If you only chase intent, you'll starve your list and cap your reach.
Full disclosure: I work for Ready, an SMS platform. Since where you invest opt-in effort changes what you eventually spend per segment, I've got a horse in this. I'll show the math and let you decide.
Why the popup out-collects checkout every time
A popup fires at the top of the funnel. Everyone who lands on your site sees it — buyers, browsers, price-checkers, people who bounced from a Google ad. That's a wide net, and a wide net catches more.
A checkout opt-in only appears to people who reached checkout. That's already a filtered, smaller pool. Fewer eyeballs, fewer numbers.
Rough industry ranges — treat these as approximations, not gospel:
- Popup opt-in rate: 3-8% of site visitors who see it
- Checkout opt-in rate: 20-40% of people who reach the consent step
The percentages look better for checkout, but the denominator is the whole story. If 50,000 people hit your site and 5,000 reach checkout:
- Popup at 5% of 50,000 = 2,500 numbers
- Checkout at 30% of 5,000 = 1,500 numbers
Sometimes the gap is wider. A high-traffic store with a low checkout rate can see the popup pull 4-5x the volume. That's the "4x" in the headline — real, and worth understanding before you decide it's a good thing.
The intent gap nobody puts in the dashboard
Here's what the subscriber count hides. A popup subscriber gave you their number before deciding to buy. Sometimes before deciding they even like your brand. Many wanted the 10%-off code and nothing else.
A checkout subscriber gave you their number while purchasing. They've already pulled out a card. They know your products, your prices, your shipping. Their opt-in carries proven buying behavior baked in.
On your first handful of sends, that difference shows up as conversion rate. Checkout-sourced subscribers frequently convert on early campaigns at roughly double the rate of popup-sourced ones — again, a rough operator range, not a lab result. They already crossed the "will I buy from this store" line once.
Popup subscribers can absolutely become great customers. But you're paying to nurture a bigger, colder list to get there. Which brings us to the part most stores never model.
The cost of holding a low-intent list
Every subscriber on your list costs money the moment you send to them. Not to store — to message. And that's where a larger, lower-intent popup list quietly taxes you.
Ready's pricing is per outbound segment, which makes this easy to model. Standard tier is $0.02/segment plus a $0.0045 carrier pass-through — $0.0245 all-in per segment. (Once you pass 50,000 segments in a calendar month, you automatically drop to $0.016/segment, $0.0205 all-in — no plan to pick, it just happens.)
Say you send one 1-segment promo (160 GSM-7 characters, no emoji) to each list, once a week, for a month — 4 sends.
Popup list — 2,500 subscribers:
- 2,500 × 4 sends × $0.0245 = $245/mo
- At a 2% campaign conversion, ~200 orders over the month
- At $60 average order value = $12,000 revenue
Checkout list — 1,500 subscribers:
- 1,500 × 4 sends × $0.0245 = $147/mo
- At a 4% campaign conversion (double intent), ~240 orders
- At $60 AOV = $14,400 revenue
The checkout list is smaller, costs 40% less to message, and produces more revenue. The popup list needs 67% more subscribers and more send spend to land in the same neighborhood.
That's not an argument to kill the popup. It's an argument to stop treating a popup number and a checkout number as equal line items on your list.
| Popup source | Checkout source | |
|---|---|---|
| Volume | High (3-4x more numbers) | Lower, pre-filtered |
| Buying intent at opt-in | Unproven | Proven (mid-purchase) |
| Early campaign conversion | ~2% (rough) | ~4% (rough) |
| Monthly send cost (4 sends, 1 seg) | $245 @ 2,500 | $147 @ 1,500 |
| Cost per order | ~$1.23 | ~$0.61 |
Where the popup earns its keep
If checkout wins on cost-per-revenue, why run the popup at all? Because volume compounds, and not every subscriber needs to convert this month.
The popup gives you:
- Reach for launches and flash sales, where a bigger list means a bigger day-one spike (mind the throughput ceiling if you're blasting fast).
- Top-of-funnel recovery — browse-abandonment and abandoned-cart flows that only work if you captured the number before the cart existed.
- Volume toward the automatic Growth tier. More segments per month gets you to the $0.016 rate faster, which lowers cost across your whole program.
The trick is not letting a cheap-to-join popup list quietly become an expensive-to-message dead weight. That means segmenting popup subscribers who never engage and capping how often you send to them — the frequency fatigue problem is real and unsubs double faster than most brands expect.
How to invest in each without wasting spend
You don't pick one. You run both and instrument them so the cost-per-revenue is visible.
- Tag by source at opt-in. Every number should carry a
popuporcheckouttag. Without it, you can't run the math above, and you'll optimize blind. - Send checkout subscribers more, sooner. They've earned it with proven intent. This is the same logic behind sending your VIP segment more — spend segments where they convert.
- Nurture popup subscribers before you pitch. A cold popup number that gets a hard-sell blast on day one unsubscribes and takes your deliverability with it.
- Confirm the opt-in. A double opt-in filters bad numbers before they cost you a single segment — and protects deliverability at the carrier level, not just legally.
- Register the right 10DLC campaign for both. A popup that promises "deals" is a marketing use case; a checkout box tied to order updates can be transactional. Getting the use case right keeps delivery from silently dropping.
Both surfaces need clean consent language and honored STOP handling regardless of source. Ready handles opt-out propagation and quiet-hours enforcement automatically, so a number that opts out on one flow can't be hit on another — worth having when you're running two collection paths into one list.
The practical takeaway
The popup and the checkout box aren't competing for the same job. The popup builds reach and feeds top-of-funnel flows. Checkout builds a smaller, higher-intent core that converts at a lower cost per order. Run both, but tag by source and stop pretending the numbers are interchangeable — because on the send side, priced per segment, they cost the same to message and pay back very differently.
If you want to model your own version of the tables above with real subscriber counts and your AOV, you can see the per-segment and Growth-tier numbers on Ready's product page, or start with 2,500 free credits and run a source-tagged test on your next campaign. The math is cheap to check. Guessing is what's expensive.