Yes , it does appear to have been used only as a label for the package. The cover cannot be opened, seem that some glue came trough and the back and front are stucked. I will follow your opinion to call it Cover, and make a note that it was stuck onto a parcel.
Based on the cut of the flaps on the back side, I would call it "cover, significantly reduced at right, previously attached to parcel". Enough has been trimmed that one or more stamps may be absent now. Being an envelope, rather than a label, there was almost certainly a letter or invoice within. I suspect it was trimmed and glued shut afterwards by a collector to "restore" its appearance.