Custom checkout experience, that allows a customer to add a product and subscription to their cart/purchase

Site URL: https://pomegranate-badger-p3wd.squarespace.com/

I'm trying to make a custom checkout process that has this flow, similar to this site: https://www.offscreenmag.com/buy/issues

1. select product, tap next

2. select subscription option, tap next

3. fill out checkout details, 

4. purchase

I'm currently using Shopify's "Buy Button" option as a code block in a SS page. But this is very limiting.  I also tried using Squarespaces checkout feature, but their Support Team confirmed currently there is no way to purchase a product and subscription in the same checkout experience.

Would anyone have any recommendation or ideas on customizing the checkout to mirror a similar checkout experience?

@bangank36, @tuanphan, @creedon