Am I required to provide copies of canceled checks?

This issue falls into the category "vendor management". I would look for a personal discussion with the "decision maker" at that vendor's company to discuss the actual reason that creates this problem and whether alternative, less costly solutions could achieve the same result. After all, you are the customer and it is up to the vendor to show goodwill in his own business interest. I would also make it clear that the most you can do for them, if you are prepared to do so, is to go through the time consuming process in handling this request. But any additional cost incurred in addition to your time would be at their expense.