Which is even better now that it’s 15% for small business

It is still outrageous because those two big ones make a huge economy of scale. They did not develop a payment method for a single app but for millions.

I think what's more outrageous is that more devs don't find it worth to do a website yourself and get a 10x cost reduction on transaction fees

There definitely is for excel. Both for consumer add-ins and internal business add-ins. I've thought about it, and even started on the idea a few times, but have given up. There are just so many components of the system that are outside of your control as a developer. Someone with the appropriate resources might be able to do something neat.

I've thought about selling my code snippet for handling payments on something like Gumroad or similar. However, it's not a great solution since people still need to sign up somewhere and then install add-on. Plus they need to subscribe on stripe with their same email that the add-on will use (since the code grabs their email programmatically). As I'm thinking through this, there could be ways to mitigate some of these issues but who knows!

There is someone that built something similar but for browser extensions (https://extensionpay.com/).