I purchased a Commodore 64 in the summer of 1983, just a year after initial release. At that time, there were not a lot of software titles or hardware peripherals available, so mail-order was the main option (thinking about it, I guess that’s still true).
There’s not a lot of information about Protecto Enterprises online, here’s a brief mention in a Wikipedia article. I most likely sent away for a free catalog after seeing an advertisement in Compute! Gazette.
Linked is the full catalog from 1985, it’s a searchable PDF.