Write a payment plugin for Braintree

[raphaelm]

[oocf]

This should be a good idea since braintree is cheaper than other payment processor.

https://www.braintreepayments.com/braintree-pricing

[raphaelm]

There's a prototype here, but we never finished it, since no-one really cared so far: https://github.com/pretix/pretix-braintree