There will be code numbers/letters on the packet somewhere. I use MSWorks version 6 (it may be a bit different if you are using something else).
In the word processor I click on TOOLS at the top of the window which brings down a menu. If you select LABELS, then select the type of labelling you want, this will give access to the various codes so you can match it with what you have bought. It is a very good idea to have your customers in an electronic database so that you can use the mailmerge facility to print on the labels. As you progress through setting up the label printing, you will see that you can insert the appropriate field names at the appropriate place on the label. I needed to mailshot all my customers once. It was a great tool. It just trundled through the database and printed off all the address labels for me to stick on the envelopes. The hardest bit was inserting the letters into the envelopes.
For what you want, you will need to look for a return address labelling facility. You could even set up a tiny database for that if you wanted (just with your own details) but it might be easier to do it from a text file.
I dislike address stamps as the ink can run when it's raining. Best to use laser printer for it.
If you have MSWorks, I could sort it out for you if you supply your contact details and the code on the labels. It would involve two small files. One Word Processor and one tiny database. If you have other software it might not be any help though.