re: We’ve talked about the tipping issue, but what about the donations at point of sale?

I don't think it's quite working that way. They are either:

A: Getting the money from you, holding it, and passing it onto the charity through the use of a liability account. This would mean they get no deduction for your contribution.

B. They care calling the money from you Income. They taking the deduction when they pay it to the charity. This means they wash and the store is right back where it was before.

A is how accounting principles say it should be handled and it shouldn't go in the income or deduction. Now this doesn't stop the corporation from pledging a certain amount be paid to the charity then using the funds from their customers to satisfy that. But they aren't getting a deduction from the customer funds.