Try this. Since I didn't buy a stamp I can't confirm that it works.
Someone could go to the "Apply or Buy" page, and then click on "Purchase Licenses". The next page they click on "Individual Purchase" and then the "proceed" button on the following page. That will put them at the login page. After they login, they will see their personal info. After they answer the residency button at the top, they click the "continue" button at the bottom. Finally you are on the page listing available products. Scroll down to the bottom and there is a button that says "previously purchased". Click there and you will see any licenses purchased this year and print buttons for all of the ones you are allowed to print for yourself. Nowadays, it is also possible to save those things as an image and carry them on your phone.