Once you calculate $25 in spare change round ups using the sources you've connected via Plaid, a purchase will be attempted using your Coinbase account. In order for the Bits app to properly work, you will need to make sure you have at least one payment method connected to your Coinbase account from this list of supported API payment methods. This means you should have as a payment method on your Coinbase at least one credit card, debit card, or ACH bank account. Keep in mind, Apple Pay, PayPal, or a Worldpay card won't satisfy this requirement

The credit or debit cards you connect to Bits using Plaid (our banking partner), are only used for calculating round ups. The actual billing is applied to your payment methods on file on your Coinbase account. This is a protection mechanism so that Coinbase is the only entity that custodies your crypto or bills you.