There are lots of ways to send packages. Stamps are used for letters, but a package tends to have various restrictions and so they get a shipping label which is supposed to be proof that you paid the correct postage without having to find the correct stamps which would be really annoying.

Business that ship a lot (which is probably most packages) don't go to the post office for each shipment. Instead they buy machines/printers/software/labels which allows them to print off the correct label, and also combines all the money into something they handle the money in one larger transaction. There are lots of options in the above for different needs, if you a shipping a package even once a week you should look into them - often you get a discount for something that is easy for you to do (sorting outgoing mail by zip code saves them a lot of money and so they will pass that on to you)

Because there are so many options you can find one option and duplicate it easily enough and it will take them a while to catch it.