Create promotional coupons

I need to send a coupon blast for our spring sale. Send the promotional email to subscribers who opened any March newsletters and include the 20% off coupon code SPRING20, expire in two weeks. Sequence the recipients so the most engaged 10% go first, then the rest. Before sending, confirm the coupon link redirects to the cart with code applied and that the subject line contains SPRING20. Deadline: send by Thursday at 09:00.

Before I send this coupon campaign to our engaged segment, make it straightforward to approve. Show the redemption deadline and the 20% savings amount right at the top, highlight whether the coupon is single-use or multi-use, surface the landing page URL and confirm the code auto-applies, and flag any mismatch between the coupon code text and the URL parameters that would break redemptions.

I scheduled a 20% coupon email to go to the top-engaged list on Thursday. The marketing lead asks whether the code SPRING20 will auto-apply at checkout, but I can't confirm from the campaign setup. I have access to the promo page and the cart developer but not sure what the e-commerce integration passes. What is the most likely cause if the code doesn't auto-apply, what quick tests prove it, and the one safest fix I can make before Thursday?

Over the last six promotions, our coupon open rates are fine but redemptions lag and developers keep saying the code was not applied. I manage coupons, creative and sending cadence. Which habit costs us the most: not testing the coupon flow end-to-end, not locking who owns the landing page, or sending too many coupons to the same segment? Recommend one immediate change to stop wasted sends and one procedural change to prevent it recurring.