$1 U.S. Postage Stamps

Another optional approach is to affix $1 U.S. postage stamps on the back of the 1040-V and W-4.

I have found that our post office does not carry these stamps. You may order them online here: https://store.usps.com/store/product/buy-stamps/1-dollar-statue-of-freedom-stamps-S_114204