Business invoice credit

Hi All,
Could anyone give me my postition on this. I am currently supplying a supermarket with goods twice a week. At the end of the month (lets say September) after issueing a statement I am getting paid on 55 days later (ie. November). I am prepared to offer them 30 days average credit ie on 15th of the next month in this case October. I wish to draw up an agreement with them outlining the terms of payment. Should their payment exceed the 30 days can I legally in charge interest (& at what rate & method of calculation) and costs? How do I stand legally?

PS By average credit I mean first 15 days + 2 15 days of the month = average of 30 days credit on 15th of the next month.

Thanks in advance for your help.

Unless you have a unique product which is vital to their existence you are wasting your time.

If they sign the agreement, which I doubt they will and it includes a late payment penalty it will cost you a lot to enforce it and by then they will have canceled your order..
To be blunt I would value the business and work on getting more shelf space and not p$ssing them off with this idea

I agree with hasta - 55 days to get paid - we should all be so lucky!