I went to my local MC, and as I expected there was a sign to the effect that the three-pack was USD 12 but there was no sign about the sale price of the 10-pack. All 10-packs still individually bore their normal price, USD 35. I picked one up to buy and went to the checkout, and the sale price came up correctly, USD 20.
MC is not always a streamlined store like Staples, BB, etc. It is big but apparently works more like a mom-and-pop store. That's its charm to me!