Understand natural cadence
Review time between purchases by product, customer cohort, and acquisition source. Use those patterns to frame a test window rather than assuming every customer should return on the same schedule.
Define the qualifying sequence
Decide which initial Shopify order makes a customer eligible, what subsequent behavior the program is intended to support, and how returns or customer identity changes are handled.
Protect long-term economics
Measure contribution, repeat timing, product mix, and reliance on continued incentives. A program should not be judged solely by how many customers use the offered value.