Hello Henya, this is a great strategic question and ultimately relies on your long term growth initiatives for the United States and beyond.
Having .ca as your country-code top-level domain can prove beneficial for local search queries. This could be a contributing factor as to why the .ca site has brought in more traffic. Using site analytics can provide you with your referring sources to confirm this.
Additionally, have .ca as your ccTLD will provide you with the ability to market to the French speaking provinces more authentically as you have the "home town" advantage of being a Canadian retailer rather than a U.S. domain.
If the U.S. marketplace is determined to be a growth opportunity I recommend maintaining the .com domain. Having the .com domain will serve U.S. and _should _ultimately convert your U.S. customers better than U.S. customers shopping your .ca site...again you can verify this using analytics. The lift in conversion should provide enough incremental revenue to support maintaining both domains. Also, if your client decides to open a brick and mortar store in the U.S. the .com site will become incredibly more valuable.
Hope this helps!
Matt
