Because the shirts will have to be returned with DELIVERY CONFIRMATION, try politely to encourage the seller to let you send a photo or scan of the labels to show him the problem. You can also ask him to cover the cost of return shipping. You can find out the cost at canadapost.ca. Use the BUSINESS rate which gives you more choices.
This sounds like a simple packing error and a sensible seller should cooperate. Politeness is a useful tool.
However, mark Day 45 on your calendar and do not let the deadline for a Not As Described dispute pass, even if you are sure the replacement is in the mail.