Confused me for a moment also! Yes, the word is "bucks". Apparently it referred to the deerskins (bucks' skins) once used to trade. Now it's a slang term, mostly in the US but we do say "bucks" here also. Though not so often.

I think what you heard is "seven bucks". A buck is slang for a dollar.

Bucks, not box.

I have no idea.