I would buy bathroom fixtures from an installer, and certainly negotiate the number of labor hours, and a discount off retail price of the fixtures themselves.
People negotiate the price of a $40 ride to the airport with taxi drivers every day. I negotiated the price of a pair of prescription glasses just yesterday, and they cost less than $300.
Funnily enough, my father does (at the local equivalent). Surprisingly, it almost always works.
Even in chain stores for hardware/DIY or white goods (fridges, washing machines, etc) there is almost always leeway for haggling to some degree on higher priced items.
He works in the automotive industry, where haggling is basically mandatory - if you don't haggle, you are going to get shafted. Its the same at builders merchants and lumber suppliers.
People negotiate the price of a $40 ride to the airport with taxi drivers every day. I negotiated the price of a pair of prescription glasses just yesterday, and they cost less than $300.
So... yes?