A few thoughts:

1) It doesn't matter if she returned it on the last day. If the return policy 90 days, that includes the 90th day.

2) If the store is "strict" about their return policy (and the register won't make the computation), they should arm employees with a calendar the returns counter, and teach the employees how to use it.

3) The OP stated the wait time was 15 minutes. I'm going to take her at her word. Yes, I've seen people exaggerate too. It doesn't mean everyone does.

4) Knowing the number of days in each month is not "common SENSE", it's common KNOWLEDGE. And apparently not common enough, or the OP wouldn't have written.

5) Nowhere does the letter suggest that the OP went "looking for a fight".