I'm in Winnipeg. When I send books down to CGC, I use Fedex and I fill out a Commercial Invoice indicating it's a non-sale shipment, with similar language to your example: "Will be returned after evaluation"
I've made 15 submissions so far and haven't yet ( ) been charged by Canadian Customs on the Fedex return shipments. (oh boy, that's the kiss of death)
CGC shipping documentation or package marking doesn't include any such reference as to how/when/why the books were originally imported into the US. I wouldn't expect it to.
Advice from the Boards I remember reading a while ago was to keep your copies of the Commercial Invoice for your shipments. If Fedex charges you for Customs/Brokerage fees, you can make the argument as to the reason for the original importation.
Hope this helps
cb