I think if you put delivery confirmation on a small First Class padded envelope (any padded envelope with a First Class postage-paid label) and maybe add a piece of cardboard for some stiffness it should make it cross country in 2 postal days. I had my first cash in mail receipt do this, but no cardboard, just padded envelope. You can print out shipping labels at the discounted Commercial Plus shipping rate, I believe, with
https://paypal.com/us/cgi-bin/webscr?cmd=_ship-now