We generally do NOT ship by USPS. If you are concerned about the shipping cost...order more stuff, and it'll be the same.
We long ago learned that our customers like to TRACK their packages. If a package takes longer than expected, the customer wants to know where it is. The cost to us of explaining that USPS has NO tracking (no, signature confirmation is not tracking) was not worth the small savings in going with USPS. Most of our shipments weigh a few pounds, so very few of them will fit in envelopes. To have a separate shipping process just for the odd decals we sell isn't worth it.
Our average order size is something like $200. Get yourself something else nice, and add a decal to it. The shipping won't change at all.
Or, if you really just want the decal, and you want it shipped cheap, just put a note in the comments field that you want it sent in an envelope by USPS for $1. If it's lost or damaged, that's on you. Sound okay?