Padloper requires that you have one or more payment modules installed. There isn't any "Padloper specific" payment module format, but I have open sourced generic Payments base module that all ProcessWire projects can use. Currently I have build Stripe and PayPal modules and also very simple Invoice module. You find more information about these modules from ProcessWire forums. Their code is available from my GitHub repos: PaymentModule, PaymentPaypal, PaymentStripe and PaymentInvoice.
Adding the Stripe payments
Install the Stripe payment module and fill the setup (secret and public keys that you can find from your Stripe dashboard). After installation go to PaymentModule settings and choose Stripe module as an active payment method. After that the Padloper checkout will find that you have payment module installed and use that. You will notice extra step where payment method needs to be chosen. If your store supports only one payment method, then you can skip that part by putting this code into your checkout template:
$checkout = $modules->get("PadCheckout"); $checkout->setPaymentModule("PaymentStripe");
This will always use Stripe as payment method.
Adding PayPal as payment method
This is pretty much same story as with Stripe. Install the PayPal module and edit its settings. Here is helpful screenshot to see relevant settings that are required at Paypal's admin (image credits geekality.net / Torleif Berger). After that edit Payment Module settings and choose PayPal as active payment method.
Adding Invoice as payment method
Repeat the above steps. Invoice is super simple implementation and you might even want to take a copy of it and customize on your needs. There isn't any settings on this one (at least yet).