My car is on auto pilot for the Post Office. I make at least daily trips to drop off packages, shipping my jewelry to all parts of the world and sending presents and goodies from my kitchen to friends and family.
One tool I couldn't be without is my 5 lb scale. I can weigh packages to calculate shipping fees. Using the USPS.com site it is a piece of cake to print out shipping labels at home. Set up an account and you can print out shipping labels for packages and avoid the lines at the post office.
The post office even supplies boxes. I keep a variety of sizes of the various shipping methods at home so I am prepared when I need to ship. You don't pay for the box until you ship, it's part of the postage fee. Flat rate boxes are the way to go if you have heavy items. Anything and everything that fits in the box will ship for a flat rate.
I've found that Etsy and ebay have great shipping systems that will print shipping labels for your packages and charge to your account. Often, the cost is less than you would pay if you stood in line at the post office. I've sold books on Amazon.com and they have the same shipping plans.
So, no need to waste time in line at the post office. Do a little checking online and you'll find it easier and cheaper to ship packages and you'll save your sanity too.