some companies needs a penalty for not keeping their delivery time, and that can only be money involved, lets say that they should not promize better that 16 weeks, and for each week over, the prize will drop 5% from the total prize.
If you pay 50% in advance, you Will either get a block! or you will be compensated with a cheaper product.

A written contract by both parties.
We have customers that demand this type of contracts for us.