[Answer]-Apply coupon to a customer's subscription based on non-stripe related actions on the site

It’s possible but may not be the best idea.

The way coupons are currently implemented in Stripe, they’re applied at the customer level and each customer can only have a single coupon. So if you’re already using coupons to offer discounts, the customer will lose whatever discount they currently have. This also means that if you’re using Stripe’s recently-added multiple subscription capability, all of the user’s subscriptions would be comped for the month.

Coupons set to “once” can also only be used (as the name implies) once—total—by any given customer, so if this a credit they can earn more than once, you’d have to continually create 100% discount coupons.

The more flexible option here would probably be to simply post a credit via the Stripe API. If you’re not otherwise using coupons (and don’t plan to), though, you could create a new 100% coupon on a monthly basis.