How to implement the ability to select and pay for MULTIPLE products on the landing page?

Question

PHP

Good day.
There is a site - sepo-auto.ru
There are two goods. Now the walletone payment system ( https://www.walletone.com)
is connected (but, apparently, not configured) here. The task is to make it so that the client can choose the quantity, payment method, delivery method ). Something like he clicks buy and gets to a separate page or a modal window appears with such fields.
Now, by clicking the buy button, the client is immediately transferred to the payment page for one product with a price (such as integration with the payment system).
How it is better to implement it? Create a separate page (modal window)? Is it possible to set it up easily, without being particularly friendly with PHP, maybe from the payment system itself? And How?)