The reason for this is the parts were either not available from the installer or I was able to get a better price elsewhere. Obviously I dont expect the shop to warranty the parts their not supplying themselves. But sometimes people have a connection or they find a great deal so they will pick up the part from another retailer as opposed to the installer.

If the consumer is willing to do the work themselves to find the best price for a product and obtain it, then why charge them extra??
The shop is still making fair money to install the product. Thats just my opinion, but everyone is entitled to their own.