Well, PPG and Dexter were really the main exception, given that they were CN's biggest hits, so CN was really cashing in on their popularity; but yes, I remember when PPG merchandise started popping up on the shelves at Walmart shortly after the series started, and likewise, I remember when Blockbuster was still in business, they had Dexter playsets (I think I may have even gotten one at one point - I seem to recall it had a large Dexter figure, smaller figures of Dee Dee and Monkey, and a machine that had different microscopic slides you could put in it).
Yeah, I actually did spend the twelve dollars on sunscreen I didn't even need just so I could get my hands on that Courage figure... and I too got the Belly Washer bottle. I think I may have even gotten that cereal top. I also had a fridge magnet someone gave me that I kept in my locker at school... I think it may have come from Poptarts, I don't remember.
I think John sold a couple of t-shirts through Stretch Films in the old days: I think one was of Eustace yelling, "STOOPID DOG!" at Courage, while the other was a Dr. Zalost shirt.