Having worked in that industry for a short period of time, and having two brothers-in-law who are long time employees in that industry, I can tell you that there are some customers with whom the distributor will only deal in cash. Certainly large chain groceries are billed, but (some) smaller groceries, convenience stores, bars, etc., especially those who have burned the distributor in the past are strictly cash customers. The driver is told that if they don't pay in cash up front, don't unload the product. Or sometimes, the salesman will make the collection in advance, but it will still be in cash.