I'm not criticizing it at all: I think it's better all the way round.
for example, you might have heard that if everyone would "just" unplug their phone chargers (when not in use) then we'd save millions (or whatever it works out as) of watt-hours a year and so help avert global warming. and it is indeed true that if everyone does something tiny and trivial (relative to a household's total consumption) like that it will add up to an amount that is much more than one household's consumption. yay! but to estimate the significance you need to compare it to the total energy consumption of the entire country. and then it becomes tiny and trivial again, and will clearly make no significant difference to global warming.
similarly here - the total is way more than $20. gosh. but if you compare it to total turnover then it is a tiny percentage, which makes no real difference to the company.
in short: big changes on a global scale require big changes on a local scale.
But don't kid yourself, adding a penny per transaction is not going to benefit the bottom line in the way your thinking.
Is this really true though? Is there some study that supports this? To me it seems simpler for the brain to round up to $20 when thinking about the price. I certainly wouldn't remember if something was $18.99, $19.95, or $19.99. They're all just $20 to me.
I always round up any price I see.
I always round down any money I have to spend.