Mendix can use Stripe, which has a beautiful back-end, but only limited payment methods. Ideal and creditcard. And there is Molly, which has all payment methods, but requires your company to already have a substantial turn-over.
Searching marketplace.mendix.com on ‘payment’: Rayzorpay and IPay88, if you are in south east asia (ASEAN) region.
You can use Stripe as mentioned, or else, go to Paypal create a developer account, an account and sandbox and then you can use the paypal personal sandbox for development purposes.
There you can find all the API’s you need.