the most convenient process involves buying a good postal scale. make sure it can accurately weigh everything between envelopes up to heavy packages. check the product descriptions carefully before making your choice.
next, set up an account at a USPS-approved provider of online postage.
don't pack it up, but do weigh the contents and packing materials together. then go to the postal program to determine the best option. once you've done that, pack it up, and print the mailing label.
(i didn't say this was the least expensive option. it's the most convenient. you can't run back and forth from the post office at 4am in your jammies. :lol: )