Literally all of your problems could by solved by buying your postage online ahead of time. The post office encourages this, and sells it through their own web

Literally all of your problems could by solved by buying your postage online ahead of time. The post office encourages this, and sells it through their own website, as well as resellers like Stamps.com. Once you've attached a prepaid shipping label to a package, you can simply drop it off at any office, and they'll ship it. No personal interaction necessary. You don't even have to wait in line.