...are quite common, for the reasons previously mentioned.
You can soometimes shop oor negotiate your way around them, with the exceptions of some(not all) plumbers.
I know of a couple of plumbers who will not install fixtures that they did not deliver to the site themselves. I know of others who will install the owner-bought fixtures, but will not warrant the installation.
Material mark-ups are much more common in remodeling than with new construction.