Should I listen to my vet and buy all the things they recommed?

I took my cat to the vet a few weeks ago and then yesterday. They recommend this advantage things for fleas and stuff and a few weeks ago i bought this stupid medication shampoo they told me was better to use than the shampoo in the stores. I found out yesterday that the shampoo is mainly for skin infections. The cat doesnt have a skin infection of any type. I feel like I wasted my money. Damn thing was $16 and the advantage thing was like $43. Did I get ripped off?