A tax "write off" just means that they get to not pay tax on a particular incoming amount of money, because it went out in a tax-protected direction.
If you donate $10 at the till, and the store passes that $10 on to the charity - the "tax writeoff" just means that the store doesn't claim that $10 as income (because it's not). Financially it's the same to the store if you donate or not.