Churn definition for non-contractual services
Perhaps you can define an "active user" (e.g., someone who has made a purchase in the last 3 months, etc. Choose a period sensible for the typical purchase cycle) and calculate the something along the line of DAU or MAU, etc. Say, if a typical active user will on average buy something from your store every 3 months, assume a user has churned if she fails to make a purchase in the last avg + 1sd days, etc.?

The other potential KPI to use is probably the CLV, but similar to DAU and MAU, it is not without its criticisms.