As someone who has done e-commerce development and supports FLOSS and self-hosting, this is something I would outsource.

It’s complex, and you can’t really handle payments yourself anyway. That requires certification.

And people really don’t like it when their e-commerce is down and may able to quantify lost business due to an outage or bug in dollars or sense. It doesn’t feel great to realize something on your end resulted in hundreds of dollars of lost business.

If the business is very small, places like Shopify have cheap starter tiers.