Clearly your customer should have taken care of this and not made it your problem.
I am not aware of 3rd parties for FedEx. They have their own offices throughout Canada. I also find them the best of them all - UPS, DHL and all the "priority" post services.
FedEx never loses anything and it's all so easily handled online. You enter your order online and then it is picked up and tracked online by anyone you list at the time of entering the order. It eliminates all the hassles and phone calls.
After many years of ordering parts to Canada from the US and either getting hit with extra duties or late arrivals I discovered the solution.
See doc enclosed. Every time I order something I email this doc to the vendor or shipper. If it is completed by the shipper and attached to the outside of the package it advises customs that the item meets NAFTA requirements and no duties are due.
This eliminates fees and speeds up the process. I get packages from CA vendors that arrive THE NEXT DAY with no special charges. Although this is not necessarily the answer for large heavy packages with value over $1600.
Hope this helps.