Open an Item Not As Described case with eBay.
If the purchase was shipped through GSP, and was damaged/partially lost in transit, you will be reimbursed in full, but not required to return the product to the seller.
Note that when GSP/PB or Canada Post open a parcel or find one that is damaged, they mark their efforts with special tape or with a letter to that effect. If that came with your parcel, say so.
If these are not there, the seller is likely to be the one who short-shipped and I would take his statements with 86 grams of salt.