Being treated like a very stupid 7 year old

I bought a really nice cooler at the Wal-Mart in Stillwater, OK for my trip to Richland, WA. This cooler had it all: a telescoping handle, wheels, cool looking cover, power cooling. It just didn't keep drinks very cold. So when I got to Richland, I went to return it. Of course the Wal-Mart in Richland had a different price for the cooler and a different sales tax. So this harried girl (it was Saturday afternoon at Wal-Mart) is trying to figure out how to ring it up correctly while dealing with a ringing phone and the loudspeaker demanding attention. So I just waited quietly. The woman behind me was not inclined to wait so quietly. To be fair, I think she was trying to help and she may have even been right. It wasn't so much what she was saying as how she said it. She told me and the cashier, but mostly me, that we should divide the total receipt by 1.083 (local sales tax) to get the original price of the item. I didn't really get the relevance of this since a) I didn't pay Richland sales tax on the item and b) the original price of the item was listed on the receipt. It later occurred to me that the lady was trying to tell the cashier how to trick the cash register, but it wasn't clear at the time, so I replied with something along the lines of "What's the point?" She explained the division part again using exactly the same words. Since I wasn't having a hearing problem, I still didn't get the point. She explained it again using the same words, only with body language and tone suggesting that I was stupid and adding "I'm an accountant, I know what I'm talking about." I assumed I must have been giving her a condescending look to merit this last remark, so I bit back a retort along the lines of "I am a PhD student in math, I know how to divide" and just gave her a look and then ignored her. To her credit she calmed down and and was extra cheerful the rest of the time in the line so I think the condescending look theory is probably right.