OP
The 18.4 rate that I stated was displayed on the checkout screen. It showed the total in pesos, stated the conversion rate of 18.4 pesos to dollars, and then showed the total in dollars in a bigger font.When you check out, the cashier can show the total in pesos or USD. The rate will be the store rate. If you get both you can do the math. The 18.4 seems low however the rate (MXN to USD) dipped low for a short time.
I agree that 18.4 is low... compared to the 20.6 we were getting from the ATM.