EDIT: I shouldn't have run to Wikipedia; much better explanation at scene.org (go figure): https://gotpapers.scene.org/?p=849

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PLK = "PostLagerKarte", literally "mail storage card". It was (abolished 1991) a way to receive letters anonymously; like a P.O. box without your name attached to it. The [Wikipedia page](https://de.wikipedia.org/wiki/Postlagerkarte) even explicitly mentions its "illegal usage in the computing scene", so GP was likely referring to his friend using their P.O. box for (illegally) cracked software.

Here's a shot at translating that Wikipedia paragraph:

> Until their abolition, PLK cards were especially popular in the cracker scene because, compared to a regular P.O. box, no personal data had to be shared with the respective post office. PLKs were primarily used for anonymous sending and receiving of pirated software on floppy disks. [citation needed imho] The PLK addresses themselves were often listed digitally in the cracktros.

pretty much a temporary "PO box" you could get anonymously. You bought a card with a code from your post office, and then people could send stuff to the post office addressed to that number, and the card holder could pick them up. Relatively popular with the cracker/warez szene.