The amount of fees and or interests they can be charged on collection accounts is a matter of both the contract and state law.
Under the Oklahoma Statutes for example:
The legal rate of interest shall be six percent (6%) in the absence of any contract as to the rate of interest, and by contract parties may agree to any rate as may be authorized by law, now in effect or hereinafter enacted.
There have been lawsuits in which the debt collectors were found to have overcharged consumers.
For that reason,I would encourage you to check with a consumer law attorney to find out whether the amount the collection agency is charging you is legal. You can contact a local consumer law attorney with experience in debt collection cases. Find out how to get FREE or low cost legal advice about debt collection from a consumer law attorney here.