Well you could still hold onto the receipt showing 3 boxes of 3 of say Trojans.
Having the receipt and the extra boxes, the SP can either check out the ones that are "unboxed" or open another box.
I will just remind myself that I am the one asking for something "extra", and not make a big fuss if she opens another box or refuses all together.
Just put any of the left over ones in my billfold, like I did in highschool, just in case I pick someone up. (aka throw them out after they are expired.)
LOL,
Have fun and be safe!