We use them at RapidRollout. They seem to be used by a lot of tech startups.
Braintree offers "transparent" redirects for maximum security. This redirects your users to their site in such a way that they do not ever see Braintree pages like they do with PayPal. The redirect just offers a way to transfer payment data to Braintree servers without going through your servers.
They also expose a RESTful API that you can make calls to from your servers (usually via their open-source client libraries).
http://www.braintreepayments.com/gateway/api
Anyway, looks like you might be able to use AJAX to access the transparent redirect API. See here:
I don't think it makes sense to limit the number of applications, event types, or team members. Cost to your customers should scale with volume of API requests.
Volume is what costs you more hosting dollars. It's also a primary indicator of your customer's need for your service, and his ability to pay for it.
Restricting access to your other features only makes your service less useful on the low end, which is where you're supposed to be winning customers. It also sets up perverse incentives that leads to e.g. piggy backing multiple real-world applications into a single app on your site, having users share accounts, etc.
You should be looking align your incentives so that you succeed when your customer succeeds. Charging for volume does that.
http://www.techdirt.com/articles/20111219/02381817122/ex-mor...