Auto-Add & Checkout Redirect via PHP

I need a small piece of code (or lightweight app/plug-in if that suits the platform better) that automatically places a product from a 3rd party site (g2a.com) in the shopper’s cart and immediately sends them to the checkout page—no intermediate steps or extra clicks.

Here’s the flow I’m after:
1. Page with input field to enter a number.
2. On submit it finds the closest value item from a page with item variants on G2A (variants = amounts).
* Closest value from bottom, for example for $22 add the $21 variant, not $23 variant.
3. Customer is then taken straight to the checkout screen on G2A.

For example -
I enter 22 in the input field
The closest value variant in the item page on G2A is $21
The $21 item is added automatically to the cart and redirected to checkout.

Any reply without " I read the description and I can make the redirect with the item already in the cart and land on the checkout page " will be ignored.