Add Test Cards#26

Similar to Stripe’s 4242 card in their test environments, test cards would allow us to test behaviors that currently aren’t testable unless the checkout total ends up being $0.00 (forever). This includes:

• Regular one-time payments
• Discounted one-time payments
• Regular subscriptions
• Discounted subscriptions forever
• Discounted subscriptions on the first cycle, then full price for next cycles

It doesn’t need to be a card per-se. It can also be a flag that can be added to a customer or anything else. The goal here is to test different behaviors in headless implementations.