Shitify Shopify checkout. They lock that payment page and there is little you can do about modifying the final (last step) page for payments. However, what you can do is take control of Customer Information and Shipping information (the first 2 steps). It requires some mild work but you can do this via one of their SDKs that provide you with checkout token. The problem is you will still need to redirect customers to that pre-designed, restrictive, feature limited, slow and incorrigible payment page…that checkout of Shopify really is pure trash. That UI aesthetic you want to facilitate to customers, you know…one of the most important aspects of ensuring a successful conversion always ends there. It’s deeply depressing.
You can however get yourself a little more in control. The token that the SDK provides will allow you reference the cart and with that you will need to look into writing your own stripe/klarna/paypal payment gateway. Solutions like Adyn are pretty good for this (especially if you’re in E.U) but this will require you to process orders outside of Shopify and this can generally become a mild headache because there are a lot of things to think about.
I have heard is rumors on Twitter that you can actually strip the checkout using cloudflare and essentially modify its contents but I am yet to really investigate that approach, though it does seem plausible. Another one is using cheerio to parse the checkout but again this will move into the territory of hackery and TTFB even when you strip Shopify checkout of JS will continue to be trash.
There is a headless with a checkout but it goes under the radar, its known as Centra - This system is very powerful but is more for corporate to medium sized and its not a SaaS like Shopify where you can just sign-up.
Anyway, hope it helps guys, sorry to be the bearer.