What everyone else said. “Change” as in “coins” vs. change as in “the difference between the price and the amount tendered.”
I shall now proceed to
explain beat the humor entirely out of this and every similar joke.
I don’t know how old you are. In the early 70s, a reasonable tip might indeed have just been the coins. Then we had almost a decade of severe inflation.
Which means by the late 80s you’d have younger people who’d only lived in the new high priced era and many ordinary middle-aged adults who were still reeling from the inflationary changes in the value of money that had been stable throughout their childhood and young adulthood.
In that era there were lots of jokes or rueful sayings about incongruous prices that just didn’t make intuitive sense any more. e.g. “$10,000 for a new car? I remember when a new house cost $8,000!?!”
So I think one way to understand the pizza joke is the 50-ish lady thinking $0.75 is a decent but skimpy tip and a 17 yo pizza guy thinking $6.75 is a generous, but not insane tip. For the same pizza. Misunderstanding ensues.
A lot of RD’s audience was the 50+ set. This was true even back in the early days. As such items about the “generation gap” or “those darn kids these days” were perrenial themes of RD’s insipid brand of humor.