Bad service from accounting department.

I am a dealer at a Service station and has had a account with Coca Cola for the past 19 years. I buy between R80 000 and R100 000 from them on a monthly basis. They recently went onto a new system and that is when the problems started. Firstly they didn't deliver for about 3 weeks and now their accounting department is a mess. They overcharged me on two invoices and when I try to sort it out I got put through to a call center where I was cut off after 25 min. If I don't pay or short pay they just don't deliver. I'VE HAD ENOUGH.