Options! There is the Buy Button, or for more flexibility the Storefront API and tools built on top: https://shopify.dev/tools/libraries/storefront-api
Not sure how much dynamic content you are using or if you need to get product information at build time etc. but this should work well.
I'd like to build out a static page similar to this: https://getdodow.io/offer-01/
As you can see at the bottom there's quite a bit of flexibility there. I imagine they are using the api?
Is there a specific api call that calls the first checkout form after you say no to their upsell?