I like to wrap keys in bubble wrap, then fold a piece of cardboard over it. With one side folded and 3 sides stapled, you have an irregular rectangle-ish mound that provides good protection, weighs little, and fits well into smaller envelopes.
First class mail is usually almost as fast as Priority, and there is a breakover point somewhere under a pound where it can be a good deal cheaper. However, Priority Mail now includes tracking at no cost, and flat rate is a great deal for heavy stuff.
Styrofoam egg cartons make great filler for larger packages, and weigh almost nothing.