I usually use the USPS flat rate boxes to ship stuff. You just grab a box from the post office, different sizes are different prices and you can ship whatever you can fit in the box up to 70 lbs
The other advantage of printing your own labels (besides cheaper rates) is avoiding the dreaded lines at the post office. I recently bought a 4x6 thermal label printer from Amzn for like $50. No ink required. Came with 100 sticker labels to boot.
I third Pirate Ship. If you don’t have a printer but don’t mind an extra stop, you can’t still print a label at your library or office supply store cheaper than going direct through USPS or UPS. If you have small boxes around the house already, that helps too. Dimensions below 6x3x0.25 (so, one side has to be above 6" on paper) don’t save you money apart from reducing weight, anyway.
The last company I was out was liquidating equipment and decided the old toner printers where ewaste. I asked if I could have it over paying a company to pick it up and got it approved. I’ve been using it ever sense and took the toner that was only for it so at this rate I’m set on color toner till 2026.