I've imported a few dozen antiques from the USA and had no issues.
The firearm must be antique in both countries. USPS will not carry firearms by mail (unlike Canada) - BUT they will allow antiques to be shipped because they are not classified as firearms. Non-firearms do not fall into ITAR, do not require export license or approval. Not all USPS postal outlets know the regulations, especially some of the smaller contract outlets. Seller/shipper should try to go to a regular USPS post office and then should have no issues.
The sender should state "pre-1897 antique" on the US customs declaration form when describing the goods being shipped.
Email your Canadian documentation from the CFO to the sender so he can print it and put it inside the parcel where it will be found if the box is opened for inspection.
If you don't have documentation you can make your own by writing a "To Whom It May Concern" letter with make, model, serial number, cartridge, barrel length, and date of manufacture, and stating that the piece has antique status in US and Canada and is therefore not classed as a firearm in either country and therefore not subject to registration or licensing. Sign it and include your name and address, ph#. Get the sender to print this letter and put it in the box where it will be found if opened for inspection.
One time, a parcel got stopped on the US side and was sent to BATF for inspection, and after 2-3 days it went back into the mail stream and carried on. I wouldn't have known it happened except for the entries on the tracker. There was no issue and arrived in good time.
Several of the parcels were held for a few days by CBSA for inspection and I suspect to confirm antique status, but arrived without serious incident or delay. One time the CBSA held a webley revolver and asked me to provide the sales invoice from the seller for calculating GST. They sent a letter, I sent the invoice back to them by fax as per their instructions, and the parcel showed up in the mail a week later. So it's probably a good idea to get the sender to put a copy of the sales invoice in the box as well. Bureaucrats like documentation.