I recently completed migrating the payment infrastructure for a major UK hosting company.
The payment sytem comprised of three Ruby web services (one using Sinatra and two using Ruby on Rails) and an AngularJS front end.
The original system used a product called Datacash from Mastercard. The Datacash service had reached it's end of life and so had to be replaced. I had a hard deadline to migrate the payment system before the existing system was turned off. Initially PayPal was selected and this work was completed on time.
Once this work had been completed it was decided that the system should be migrated from Paypal to Stripe in line with other parts of the business.
One of the big challenges with this project was that some of the code had not been changed in over ten years so a lot of it had to be upgraded to be compatible with new payment APIs.
In addition to this I had no previous experience of AngularJS so modifying that site was quite challenging.