Item Listing Price vs. Buyer Paid Price

I sold an item earlier that had a listing price of $24.95, but when I viewed the order details it showed the buyer paid $22.45 - a 10% difference from the listing price.

If the buyer uses some sort of outside deal, does it show us? I ask, because I have a promotion where if the two or more of the item are bought, they get a 10% discount, but they only purchased one.

I looked through my other single sales of similar items under the same promotion, and none of them had the 10% difference in listing price and buyer paid price, so that was a relief...but is there some weird workaround that buyers can do to "trick" my promotion? Or does the buyer using some outside deal, etc. just not show up in order details?