Do you support 3rd party checkout like Checkout-X?
No, we 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.
Limitations of Shopify Draft OrderLimitations of UFE
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