How can I get each purchase to succeed without needing to feed it real
money from my credit card/paypal each time?
In testing phase, the new API does not provide a class that you can use to simulate license info during testing. So, to simulate the purchase process, you need to buy the real product.
If you just need to get the product to ensure the correct feature or service could be unlocked after the add-on gets purchased, you could generate promotional code through Dev Center to get your product without feeding money:
Also, note this in that document:
In some cases the customer may see a Buy button instead of Install,
even though the app was successfully redeemed via the promotional
code. The customer can click Buy to install the app for no charge.