+0
Suppose there is some ketchup for sale. The ketchup is bottled and the bottles are packed into boxes. Each box holds six bottles. A store sells the ketchup in such a way that a consumer must buy a full box of six bottles of ketchup instead of the individual bottles. To describe this situation, I thought of a few sentences:

"The ketchup is sold as six-bottle boxes."
"The ketchup is sold as boxes of six bottles."
"The ketchup is sold as six bottles per box."

It seems all three are usable English, as all three patterns are found on google. But are all three good English?
Comments  
I would most likely say "The ketchup is sold in boxes of six". We can guess "bottles" from the context, but to make it clear you could say " The ketchup is sold in boxes of six bottles"
So, this would be wrong:

amazon :
"Plastic Prince - Heavy Duty 42 Gallon Contractor Bags - 3 Mil - 32-Inch by 48-Inch - Sold As 20 Bags Per Box."
Teachers: We supply a list of EFL job vacancies
Let me clarify; These two sound fine to me "The ketchup is sold in six-bottle boxes." "The ketchup is sold as six bottles per box.", but more naturally in BrEng, I would say "The ketchup is sold in boxes of six"