Stripe is by far, in over a decade, the simplest method of online payment processing I've ever run into. You can add it to your app in a trivial amount of time, customer service is great and the documentation is clear and easy to follow.

The only negative I can think of is there are cheaper options out there, but at 2.9% + 0.30 per transaction, its reasonable enough for most of the clients I've implemented it for.