Nothing stops fiat merchants today from charging excessive fees. They tend to not do it because its bad for sales.

Imagine if every credit card terminal had to be built and maintained by the shop. They'd probably all break in less than a year.

Furthermore, imagine if every shop had to secure their own intra-bank settlement contracts so the money could actually get to their account. I imagine the local butcher/baker would have a difficult time figuring that out.

This is why no pleb store is gonna fart around with sovereign LN. They'll just outsource it to a bank or payment processor like the do today with fiat.