Silly question but in a company that size, wouldn't it be more economical to provide all staff with the same refreshments?

Across 3 buildings we have various vending machines, coffee stations, etc but all offer the same.  There is no difference made.

I think that's the first problem.  Once you address that the rest will fall into place. 

Realistically, how on earth are you going to police this and stop someone from filling up a thermos?  Personally I wouldn't be that greedy but that's just me.