I read somewhere (HN?) that a guy (an engineer who somehow got temporarily involved in running a restaurant?) got a restaurant a huge (70%?) discount on meat by buying a year's meat upfront on a predictable delivery schedule.

Your vendors hate 90 day payment just as much as you hate when your customers do it, and they are covering for non-payment, late payment, and difficult customers by raising their prices. Crucially, they have to charge you the "bad customer" price because they don't know you, and then your incentive is to be a bad customer because it doesn't cost anything extra for you.

The restaurant owner thought it was crazy but couldn't argue with the cost reduction. The moral of the story was that if you'd just communicate more you may find a mutual benefit. The restaurant loved the vendor but the vendor also felt the restaurant was their best customer even though they were getting 70% less money! - because of the predictability, less need for inventory, better cash flow.