Subpoena for documentation

My company is in Canada and in our computer logs, we record electronically text conversations of our clients. We offer our services in Canada and US. Our lawyer has just received a subpoena (on our behalf) from the US requesting the conversations between 2 particular clients to between a time period to be disclosed.

The effort to produce this information is quite heavy, and was wondering if the requestor even has jurisdiction here. Also, can we request compensation for the effort it takes to retrieve this information? Can we deny providing this information? Do we have to notify the clients that we will be discolsing this information?