For whatever reason this is hard to find. Paypay.com offers delivery confirmation from USPS for 18 cents (vs. 60 cents from USPS). You just have to buy your label from PayPal.
https://www.paypal.com/us/cgi-bin/webscr?cmd=_ship-now&info
You’ll need to have a PayPal account and it will ask you to log in before you’ll see the shipping options.