Can a business selling to my business tack on interest andlate fees if there was no signed aggreement that established the terms for doing business?

I have a business which has 3 or four primary suppliers that I have ongoing relationships with. Only with 1 of them did I sign an agreement stipulating the terms so that I would be allowed to sell and install their product. The others do extend me an open line of credit that gets paid as I get paid on various jobs that they have shipped to, but usually I always have a balance based on the revolving nature of the business. Never have any late fees or interest been charged, as our ongoing relationship is what is important to them. One supplier, however, had been hitting me with late charges, and when I mentioned it to him, I was told "that is just the accountant, don't worry about them). I have an e-mail from the owner that said he would get them taken care of. That never happened, and now the owner of this company has turned over my account into collection. The balance consists of interest/late fees, and credits that I was due but never received. What can I do?