[BillyCoxDodgers3B]

Rick brings up an important point. The Printed Matter route is best served for smaller shipments that fit into a flat envelope. 40 sets of "Printed Matter" is both hilarious and suspicious!

Since you're dealing with so many cards, I'll safely assume that the value is on the lower end. In this instance, you're best served to just say they're "cards" or "trade cards". Just avoid saying "Baseball Cards" while finding an honest way to describe them. You could also say "Ephemera--40 sets of cards". While still honest, "ephemera" might throw off an otherwise tempted worker from stealing your shipment.

Also, I have to hand it to you for finding a Canadian willing to pay what will likely be a fairly high rate to ship so many sets their way! (It's becoming borderline impossible to find American sellers who will ship north of the border--even for one nearly worthless item--so thank you for being so willing.)