Contact the seller again and attach a photo of the broken item and the packaging.
Ask again for a full refund.
If the seller refuses, start the Item Not As Described dispute.
If the seller does not refund on that, given that you have sent him the pictures, PP will require a return with Delivery Confirmation. This is Expedited Parcel to anywhere in North America.
Use PP to buy your shipping label, you should get a discount from PO counter prices. In addition, this gives a little more proof that the item was returned.
Since it is already broken, you may not need as large a package, but do be careful that no broken glass can escape to hurt the PO personnel.
PP will refund you when the DC shows on canadapost.ca. and usps.com.
They will then go after the seller for the money, and he will get a black mark on his selling account.
You have up to 60 days to leave feedback. More important are the Detailed Seller Ratings you can leave at the same time. The normal rating is Five. The lowest Rating is ONE STAR,