I'll take your toilet/shower example and point something out. I live one plot over from the Merrimack River. That's where (eventually) my wastewater ends up. (It takes a rather circuitous route to get to the treatment plant bofore ending up in the river, though). 3 miles downstream is Massachusetts. That's interstate commerce. What my town charges me for that service depends on what standards they have to treat the water since it's going to head into Massachusetts and, after that, out to the Atlantic.