I can't help thinking you are going about this the wrong way: the schema you describe only works if all products come in boxes of three strips of 10 items - and in the real world that is generally not the case. Medication for example will often come in strips of seven, ten, fourteen, twenty eight, or thirty-one, and be boxed in two strip, three strip, or four strip depending on the "Normal" number of doses per day.
And most establishments want to "get rid" of the oldest stock first, so loose strips will probably be preferred over whole boxes to keep their count down.
I'd suggest that you re-think it, and include strip and box stock levels, along with strip and box "contains" counts:
item_id = 10000
description = "Something shiny"
BoxContains = 3
StripContains = 10
BoxCnt = 3
StripCnt = 0
And then write your SP to accept a "total sold" and use those numbers to update the DB.
It'll be a lot less trouble in the long run ...