Wouldn't that affect, for instance, eBay sellers who need to ship their goods? You'd need to bump parcel mail up in cost in order to stop people using that instead. I've got small items from fricking China for less than $5 total including shipping!
Just set a 5 dollar rate for standard envelopes and slide everything else that was previously less that 5 dollars up a bit. Things that were previously 10 dollars or more would stay the same.
People who are running businesses that involve sending legitimate mail cheaply will just have to start collecting larger shipping fees. It needn't affect anyone shipping anything but small things.
That sounds like a good idea, but I'm afraid that you'd be left with credit card and insurance companies who could afford those high rates. Your local small mom-and-pop shops would be left out.
We could jack the price of a stamp to $5, it would still be a reasonable price, and nobody but the stingiest of cheap people could possibly complain. If mailing something isn't worth the price of a hamburger to you, then you shouldn't be mailing that something.