Granny, we do it all the time. We are lucky to have a postage scale from our business so we are able to weigh our packages, then look on line for size and weight at the USPS site. Once we figure that out, we mark it on the package where the postage goes, write a check to postmaster and put it on, in or hanging on our mailbox. They pick it up for us, and if we write a check for too much or don't give them enough money they let us know and we make it right with them if necessary.
We live rural 8 miles from the nearest town of 1300 people so they know us but ask your mail carrier.