Does it work with Shop Pay / GPay / Paypal buy now buttons?
No, it won't. Because of some limitation from Shopify.

What exactly is the limitation?

Our app uses Shopify Draft Order API to apply discounts to the upsell/cross-sell offers.
Unfortunately draft orders works only in Shopify native checkout.
You may find the below articles helpful.
Copy of Limitations of Shopify Draft OrderCopy of Limitations of UFE
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