Billing in general is very hard. Especially subscription billing. Just from my experience in building billing and also later on using a third party system for a

Billing in general is very hard. Especially subscription billing. Just from my experience in building billing and also later on using a third party system for another projects, I can mention so many edge cases: 1. Grandfathering 2. Upgrading across different length memberships (monthly tier 1 to yearly tier 2) 3. Crossgrading across different lengths (monthly tier 1 to quarterly tier 1) 4. Offering free trial to tier 2 when user has tier 1. 5. Promotional offers to non-paying users (half off first month) 6. Downgrades 7. Applying coupons to accounts (customer support wants to offer a specific user a free month of service when user has a quarterly subscription: billing schedule change) 8. Differences in tax regulations across countries (VAT vs whole value tax) 9. Differences in display prices (in some countries displayed prices should include tax) 10. Handling currency exchange rate changes.