First of all, there isn't much of a markup on strings to begin with (unless you are buying from a music store

).
Secondly, padded mailers aren't free. And last but not least, a package of strings doesn't just magically jump off the shelf, put itself into the envelope, or self address and stamp itself either. Nor does it drop itself in the mailbox. What do you think the "handling" part is of shipping and handling? Products don't ship themselves.
For a place that sells ONLY strings and has the widest selection of strings on the planet, and given the fact that for this kind of business a flat fee for shipping makes the most sense, I really don't see what the big deal is with $7 to get your strings to you in a few days. My guess is that most would easily piss away (literally) that much money in drinks at a bar.
Time is money, and unless you run your own business, you won't understand how getting merchandise out the door costs you money (not even including the actual shipping cost), no matter how little time or effort it actually takes you. It takes just as long to ship one string to you as it does to ship a whole set. When you run a business, you are either making money or losing money. There is no such thing as breaking even, because if you aren't making money you are wasting time not doing so. Successful business owners know this. Selling and shipping strings for a living is not the same thing as you selling and mailing a single set to a fellow TB'er one time. Do it thousands of times and see how much time is wasted doing so.
I never begrudge anyone what they charge for what they do. If you don't like it, don't buy from them. Funny how people complain about the cost of something until that something is the product they are selling. I'm guessing you'd feel differently about it if you owned juststrings and were making a living from it.