BTCPay Server is fantastic, though I have things I don't like about it so much: size & bloat, language (C#), installation mess. These differences lead me to creating a lightweight payment processor a few years ago - SatSale (also not sufficient for our current needs).

For us, we really wanted some custom features and I like having absolute control over the system:

- Minimum order quantities (doesn't make sense to buy a single Frostsnap device).

- Avoid PHP platforms (vulnerabilities galore)

- Promo codes with custom rules and application

- Database design (for customer privacy)

- Shipping cost

- Eventually fiat integration

You can probably achieve many of these with BTCPay, I haven't looked in a while (and perhaps should have).

A diversion from building the product itself yes, but I'm really happy with its current state and do not regret it since it should be sufficient for a few years at minimum. Maybe I'll open source it at some point, but would require serious manual adaptation for other products.