Writing it off as a loss isn't useful.

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)

Why wouldn't the write off be useful? I think your formual needs to add "+ ($Y x .3)" for tax deduction if you frame as promotional or other tax write off strategies.

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.