SMS opens faster than email, but that is not the whole back in stock story

Every merchant asking “should our restock alerts be SMS or email” already knows the folk answer: text messages get read, email gets ignored. That folk answer is mostly true and also not the whole picture, because the two channels differ on more than attention. They differ on cost per send, on what the law requires before you can send at all, and on how each one fails when something goes wrong. Here is the actual comparison, not the pitch a texting app gives you.

SMS wins on attention, and it is not close

Text messages get opened. Reported SMS open rates run 90 to 98 percent, against an average email open rate of 28.6 percent, and SMS click-through rates of 21 to 35 percent against roughly 3.25 percent for email (reported by Omnisend). Conversion follows the same pattern: 21 to 30 percent of SMS recipients complete a purchase after a text, versus about 12 percent for email, per the same roundup.

Worth a caveat before you get too excited: a chunk of that SMS “open rate” reflects that texts show up on a lock screen whether or not anyone reads past the preview. It is still a real attention advantage, just not proof that every recipient absorbed the message the way an email open (which requires actually opening the email) implies. For something time-sensitive like “the shoes you wanted are back,” that lock-screen visibility is genuinely useful. The person does not have to open an app to know the item exists again.

Email wins on cost, once your list has any size

This is where the SMS pitch usually goes quiet. Shopify’s own SMS pricing runs $0.012 per message sent to a US number, $0.025 to Canada, and up to $0.13 for some European countries, with messages over the character limit billed as multiple segments (Shopify’s SMS pricing page). Third-party SMS apps land in a similar range: Postscript’s published tiers run $0.007 to $0.009 per SMS plus carrier fees on top, before you add MMS or a toll-free number fee (Postscript’s Shopify App Store listing).

Run the math on a restock with a five-thousand-person waitlist. At a cent a text, that is fifty dollars, every single time that product comes back, on top of whatever monthly platform fee you already pay. Email has real infrastructure costs too, but the marginal cost of one more email to one more subscriber rounds to nothing on any standard email platform. For a product that restocks often, or a merchant with a large list, that difference compounds fast. SMS is the express lane, and express lanes charge a toll per car. If you want to see how that per-message toll stacks up against each app’s monthly fee, we did a line by line pricing teardown of the six most-reviewed back in stock apps.

The opt-in law is not symmetric

Email marketing consent, in the US, runs under CAN-SPAM, which is relatively permissive: you need a way to unsubscribe and you have to honor it, but the bar to start emailing someone is lower than most merchants assume.

SMS marketing consent runs under the TCPA, and it is a different animal. You need prior express written consent before sending a marketing text, not just an email address collected somewhere. Violations carry statutory damages of $500 to $1,500 per message, per recipient, and this is not theoretical: a retailer settled a TCPA class action for $4.42 million in March 2025 over unconsented promotional texts, and as of April 11, 2025, the FCC also requires businesses to honor SMS opt-outs within 10 business days through any reasonable method the customer uses (reported by MoEngage). A “notify me by text” checkbox that was not written clearly, or a consent record you cannot produce two years later, is the kind of thing that turns a restock feature into a legal bill. This is not a reason to avoid SMS. It is a reason to have your consent flow reviewed by someone who actually knows TCPA, not just configured by an app’s default settings.

They fail differently, too

Email’s failure mode at restock volume is well documented: mailbox providers watch for sudden bursts, and Google’s bulk sender guidance specifically warns senders to “avoid introducing sudden volume spikes if you do not have a history of sending large volumes,” with spam rates above 0.30 percent triggering real delivery problems (Google’s bulk sender guidelines). A popular restock blasted to a big list in one instant can trip that exact pattern, landing a chunk of the batch in spam for a reason that has nothing to do with what the email says. We went deeper on why restock emails fail at the exact moment they matter, burst sends included, if you want to check your own app for it.

SMS has its own version of this: carrier filtering, throughput limits per number, and the same kind of scrutiny on sudden volume, though the mechanics differ because you are dealing with telecom carriers instead of mailbox providers. Neither channel is immune to “we sent it and it did not arrive.” Both need pacing and monitoring, not just a send button.

So which one, actually

If you have the list size and margin to support it, and your consent is genuinely clean, SMS for your highest-demand restocks (the drops that sell out in hours) is a real, defensible choice. The attention numbers back it up.

For everything else, meaning most of a typical Shopify store’s catalog, email is the more sustainable default: near-zero marginal cost, a lower compliance bar, and infrastructure that scales without a per-message meter running. Plenty of merchants land on both: SMS for the handful of items that justify the toll, email for the rest of the list.

Where Kelso fits

Kelso is built for the email side of that split: two-path restock detection (a webhook for speed, plus a reconciliation poll that catches what the webhook drops) and paced sending so a big restock does not trip spam filters at the exact moment it matters. It does not do SMS today, so if your plan depends on text alerts, you will need a dedicated SMS app alongside it. If email is your restock channel, or you are deciding whether it should be, that is the part Kelso is built to get right: not just sending the alert, but making sure it actually lands.

The short version

SMS gets read faster and converts higher per message, and it costs real money per message and carries real legal exposure if consent is not airtight. Email costs close to nothing per send and asks less of you legally, and it has to fight harder for attention. Neither one is the correct answer by default. The right answer depends on your list size, your margin, and whether your opt-in flow could survive someone actually checking it.