Bad service COCA COLA

My company has had an account with ABI for the last 18 years. I pay my bill within 7 days. Someone in accounts didn't do their job and they have been harassing me for payment for an invoice of almost three weeks ago. Even after I've sent them a proof of payment where the invoice was paid within seven days. They are holding my delivery for clearance although it is their fault. They have a monopoly in SA and there is nothing we can do about it.