Don't get caught using media mail or library mail rates to ship anything but that which is allowed. The fines are huge!
Also, poly mailers are stronger, more waterproof, and cheaper than tyvek envelopes, but if you want stuff to be truly waterproof, a sealed inner bag is necessary. Ziplocs are a cheap alternative if you're not shipping a lot.
A good place to get shipping supplies is www.uline.com.