I'm not well versed in labor law, but if there's a mistake, there's always a procedure for disputing it. And, if they are at all like the IRS, they may be willing to write off the late charges if you are paying the original debt. The starting place would be to figure out if their figures are accurate.
How much are you disputing and how long ago was the mistake, or miscalculation, if you know.
I may be guessing or not licensed in your state. No atty/client relationship exists.