If we figure two-fifths of cash transactions need to be rounded up and the store is losing an average of 1.5 cents each time, their expected losses would be around $2,000, yeah?
They're rounding all cash transactions down to the nearest nickel, so an average of 2 cents per transaction, 3.4 million customers, gives me $68,000 assuming each "customer" makes a single transaction per year. If they mean that there are 20 million unique customers, not 20m transactions, then the a long tail of customers who make frequent small transactions in cash could make their claim check out.
Without a write off, their income is $X (what they actually collected), with a write off, their income is $Z (what they should have collected) - $Y (what they didn't collect), but $X = $Z - $Y. There's no material difference between counting what they actual collect as income vs what they should have collected minus the goodwill discount. Unless there's some specific tax justification (maybe accounting differences could justify remitting less sales tax overall and retaining more of the funds, etc)
It won't be the same as what they would have collected without rounding, but it will be better than if you didn't write off anything.