Free shipping doesn't mean that the shipping service provider is giving them free service just because it's a large order/package. Instead, the seller is giving you free shipping because of the amount that you have ordered, or the total amount of money that you have spent. It's a common sales tactic to get you to buy more than you usually would. Sure, it cuts out some of the profits from their sale, but it keeps you coming back and making larger orders which in the long run makes them more money than not offering the free shipping in the first place.
Personally, I have had good results buying things from The Perfect Bass, Just Strings, and the Bass Emporium.