Help understanding promotion rebates

Im brand new to selling and all this Amazon stuff and today I made my second ‘sale’, my product goes for $12.95 and I am using FBA. The customer somehow applied a $12.95 promotion for the full price of the item (I dont even fully understand what a promotion is) and then I was charged $4 for amazons handling fees? I dont get how that is fair, I basically just paid $4 to lose a unit? Somebody please explain im so lost lol