Many people try and save $. Its normal and part of keeping all parties honest. I can think of many things I would buy online and not think twice. The last time I tried to purchase a part for my craftsman rider mower they would have to order it in. Well if I got to wait I might as well save half.
Products that require a service are much harder to seperate and cause all kinds of problems. Case in point. My mechanic will rarely install parts he doesnt buy. Is it the 10% markup? Not really. But when he recieves the wrong part and has to hustle the right part how does he cover his time? Make you do it? It could have been the parts co or it could have been you. Afterall hes the mechanic not you.
Perhaps a better comparison. My chiropractor wants to go all cash no insurance. He wants to just get paid for his time. Its a service to you to deal with all the shysters in insurance that he has to cover. He has to charge More for the same time just to get your/my business. He is in the exact opposite situation as you! All he wants is his time and not the rest of the hastle.
If you took your animal to the vet because it needed imediate attention and the vet didnt have all the supplies to handle it wouldnt you be mad? I would.