Because you will be pay $40 in shipping sending it back to your maybe authorized dealer. Even IF they pay for the shipping for you, you are out of music for at least a week. When I was 16 and broke, I didn't care I wanted cheap as possible, so I understand. But I am not so broke any more and I like having my product when I want it, don't have to worry about damage in shipping, and easy returns/swaps if I don't like it or if something happens. A few bucks is worth that premium. I don't know about all shops, but if you buy from us and it is a legit warranty claim we will swap on site. If you bought the extended warranty, we will swap on site even if you smoked it.
Any place I have seen that puts tax into shelf price is a ghetto back door type of place. No matter what you pay tax. They are just discounting that amount for you. Or they aren't reporting their earnings, which again is a place I would not want to buy from or have work on my car. I have no problem at all negotiation a price, but I absolutely will not sell for cost unless I am getting labor too.