I checked a couple of web sites & they said the coll. agcy. cannot add interest unless it was authorized in the orig. agree. & permitted by state law. I don't know what the state law is in NY. Hopefully, OP will come back & let us know.
I checked the fed. FDCPA & it says: A debt collector may not use unfair or unconscionable means to collect or attempt to collect any debt. Without limiting the general application of the foregoing, the following conduct is a violation of this section:
(1) The collection of any amount (including any interest, fee, charge, or expense incidental to the principal obligation) unless such amount is expressly authorized by the agreement creating the debt or permitted by law. sect. 808 #(1)

