Ask HN: How would you handle e-commerce with static sites?

If you were setting up a site to sell services or other non-physical products, and using a static site approach (maybe using Middleman?), how would you handle e-commerce?

Am I missing some clever approaches that are ready-to-roll that include coupons and order editing? Or is the correct approach to just roll your own with some storage-as-a-service backend like Firebase or Parse?