How would you implement API calls towards a payment solution

Hi!
So far I've been using Stripe and Paypal + Rails gems to make payments for my apps.
These days, I am considering changing to a new vendor, with better prices and services fitted for my need. So my question is this:

How would you implement proper API-calls towards these endpoints, without any gems?
I can not find the vendor in e.g. ActiveMerchant. The calls needed from my Rails app should be fairly basic with authentication. JSON is supported.

Any general payment/api-call gems for Rails?
Are there any JS-libraries I should use to help me out?
Have you seen any good tutorials discussing this type of general API request?

Thanks a lot for any answers.
ps: Loving the videos Chris!