So, I've now setup braintree, and enabled it as the gateway. I tried a transaction, which takes me to PayPal (Which is probably right, braintree is just BackOffice for PayPal, right). The amount in the transaction is 10$, regardless of what I defined for the level.
I do the clickety clickety thing on PayPal, and it returns me to my discourse site, showing:
No transaction recorded in braintree, no adding that account to the corresponding group.