"He who marches out of step hears another drum." ~ Ken Kesey
in this day and age with the Internet and most vendors selling the same stuff everywhere, I would tell them that you're going to switch if they don't do what you want.... if they value your business, they'll figure it out, if not... there's always someone else who would like your $$$.
Find a broker.....I used to have a 6.00 fee for a Fedex pickup....Broker does huge volume so I no longer get charged the fee.....In most cases a broker will end up lower cost than anything a small shipper can get on their own....
He said "Supplies"... so it sounds like he's talking about chemicals or something similar.
The threat of taking his business elsewhere is useless if he is buying specialty garments direct from the manufacturer. The other thing is that there is no such thing as free shipping. Some people spend too much time o searching for free shipping versus getting top tier pricing.