What happens to the money you donate at the cash register? 

This is where you round up your bill to give to a charity designated by the retailer, and the donation amount appears on your receipt. The store serves only as a collection agent for your gift. Assuming the business is following the law, it will not include your donation as part of its business receipts, or income, nor will it claim the charitable gift as an expense. 

In other words, your gift has zero impact on the store’s income taxes. Keep in mind that the store chooses the receiving charity, so make sure it is one you can support. As a customer, the donation will appear on your receipt and you can claim it as a charitable deduction when you file your income tax return. But you probably won’t.