Collection Agency won't provide info by mail

Hi,

I have a car that is listed as a charge-off. I still get statements from the credit union that financed it which states that I owe around $1800. The collection agency that has been assigned to this case tells me I owe somewhere in the neighborhood of $4500. The difference, they say, comes from fees and interest. When I ask them to send me a breakdown of these fees in writing by mail, they flat out refuse. They say they do not do business that way. Huh? My car isn't even worth that much. If I were to just pay the credit union directly, can the collection agency still come after me for the fees they've tacked on? At this point, I'd rather they just repo it, sell it at auction, and be billed for the balance that the credit union says I owe.

Btw, yes, the payments I've made to the collection agency are listed in the statements from the credit union.