And that was a rude way to strawman my point that your "rule" is not about 80 cents, but about conceding to every such demand, and is therefore impractical as actionable advice for everyday life. (Making you look generous, and me unable to afford the tip on one coffee shop muffin, on the other hand...)

Edit: I would say it is indeed unreasonable to expect people to increase payment on any good by 20% merely on the chance that they might be expected to -- few people can afford 20% slack on every purchase. Also, I only brought up VS because I'd seen you question the utility of the concept and this seemed illustrative.

But what if the cost of the muffin itself went up 20%? Would people stop buying it? The shops around here don't even list prices for muffins. It could be $3 or it could be $5. Customers seem to think they can afford it either way.

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.