They will also register a sale with a supplier for a big project which will give a much bigger discount from the supplier and the 3% will be on standard price. This could add another 10%-15% to a sale.
It's why the first supplier you ask to quote should be able to get much better prices for a big enough project (or large quantity). Only one supplier can register a sale normally.
It's also why suppliers try to encourage you to get demos or speak to suppliers via them and not direct.