Klaviyo vs Postscript for Shopify SMS marketing

Consent, quiet hours, and purchaser suppression

Every SMS decision on this page sits on three operating controls. First, consent: TCPA plus state-level rules (California, Florida, Washington and others add their own wrinkles) require a documented opt-in source, timestamp, and disclosure text for every number — keyword, checkout checkbox, popup, and QR captures each create different expectations. Second, quiet hours: no promotional texts before 8am or after 9pm in the recipient's timezone, enforced by timezone-aware sending rather than a single store clock. Third, purchaser suppression: recent buyers exit promotional flows same-day across email and SMS, so nobody gets a cart nudge for the order they placed an hour ago.

Suppression also covers opt-outs (immediate, all channels), complaint-prone segments, holdout groups, and subscribers whose capture promise does not match the message you are about to send. Losing suppression history during a platform switch is the costliest migration failure — it manufactures complaints from previously happy subscribers. Review SMS consent basics and the TCPA and state-law guide before your next capture change.

• Log opt-in source, timestamp, and exact disclosure text at capture — never reconstruct retroactively
• Honor purchase exits same-day on shared profiles across email and SMS
• Enforce quiet hours per recipient timezone, including daylight-saving edge cases
• Keep STOP/HELP replies working on every sender ID; test them monthly

Margin math and app costs

SMS looks cheap per message until you subtract everything attached to it. A useful worksheet: incremental revenue per send, minus platform subscription, minus per-message carrier and surcharge fees, minus discount cost on redeemed codes, minus refunds and support replies. What survives is the number that matters — contribution margin per send. Stores that skip this math routinely pay more in app fees and discounts than the channel returns.

App costs also compound quietly: contact-tier growth, SMS credit packs, carrier pass-throughs on toll-free or short-code traffic, and add-on fees for premium features. Model the peak month (Black Friday, a product drop), not the average month, and compare that peak across stacks — see SMS pricing economics for the full worksheet.

Pricing deep-dive: what to verify before you commit

Pricing on this page is directional, not quotable — plans change often and carriers adjust surcharges several times a year. Before committing, check the official pricing pages for each app on your shortlist and confirm four things in writing: what the base tier includes, how overages are billed, which carrier fees are pass-through, and what happens to billing when your list grows 2x or 10x.

Then run a 30-day pilot at real volume with a holdout group. Attribute revenue conservatively (platform-reported attribution overstates lift), and report margin after all the cost lines above. A platform that wins at your current list size can lose at twice the size — re-run the model quarterly, and revisit the full app rankings when your send mix changes materially.

• Confirm base-tier inclusions, overage rates, and carrier pass-through fees in writing
• Model peak-month (not average-month) cost at 2x list size
• Pilot 30 days with a holdout; report margin after discounts, refunds, and support
• Re-check official pricing pages each quarter — numbers on review sites age fast

Decision table: which layer owns the job

Use this table to assign each message job to a channel and tool before shopping for software. Most Shopify stores fail by buying one platform to do everything, then letting frequency and consent discipline slip across all of them at once.