I was referring to the case where all expenses go up 20% and claiming that most people's budgets couldn't handle it (cf. stories about not handling a $500 unexpected bill). This is the relevant case to compare to if you take tptacek's advice seriously and operate your life on it -- you can't just say "Hey, it's 80 cents, obviously people don't turn away from that". You have to ask, "what if I responded to all requests for a tip at electronic PoS this way?"

Goods do fluctuate in price day-to-day, but in an uncorrelated fashion; that kind of fluctuation doesn't have the same effect since some are up and others down.