I order Flat Rate envelopes (and other envelopes and boxes, too) straight from the USPS. They deliver them to my door, free of charge. The ones I get are not pre-paid, although pre-paid ones are available too. Here's the link:
https://store.usps.com/store/browse/...S_PriorityMail
Then, when I've got a package to ship, I've got my envelopes and boxes already on-hand.
I print my postage through PayPal since I've already got an account with them, but you can also print postage straight from the USPS website. Tracking for Priority Mail is included, and now they're including insurance as well (up to $50, I think). You don't have to buy expensive adhesive-backed labels. You can print them on regular printer paper and tape them securely to the package (just don't tape over the bar codes). Below are links for both:
PayPal:
https://www.paypal.com/us/cgi-bin/webscr?cmd=_ship-now
USPS:
https://www.usps.com/ (once there, click on "Ship A Package", then "Print A Label With Postage")
After the label and postage are printed, I can request that my mail carrier pick up the package on her normal route. Here's the link to request a free pickup:
https://tools.usps.com/go/ScheduleAP...n!input.action
I don't even have to leave the house, and printing the postage online usually saves a few cents, because on many things the online price is a little cheaper.
(Edited to add: For example, when I mailed my completed blocks back a few days ago, my padded Flat Rate envelope costed $5.05, and that included Delivery Confirmation and $50-worth of insurance.)