The supply house should be quoting retail prices to your customers and selling them to you at wholesale prices.
In my opinion they are walking on you.
They may or may not be. Either way in every situation it was customers option where to get the lights, I merely suggested they check out both of my suppliers, and next thing I know I receive lights I did not know I was going to receive. Their price Vs the profit margin I would have added if I ordered them myself is less profit for me, but it is profit I was not even anticipating in the first place and required no work from me except billing the customer and paying my supplier bill at the end of the month. I was going to get paid to install them no matter where they purchased them.
I have been suprised by many customers that opted for this route figuring they would have bought their fixtures at the Home Center because they generally are less price there. Either the sales people at the lighting showroom are good salesmen or customers have better sense of "you get what you pay for" than I sometimes give them credit for. Of course fixtures are like furniture or appliances, they are things they see and use everyday and are worth spending more money on, decora devices also fit in this category. Whatever gets enclosed in the walls, buried in the dirt or placed in the mechanical room needs to be as low cost as possible.
I do not bid or estimate fixtures for residential outside of recessed lights and linear fluorescents unless there is a specified fixture. I may include an allowance depending on bid or estimate conditions but that still leaves the possibilities wide open of what will be installed there.