IIRC, the only pre-paid labels that a Post Office can accept for items going abroad are the International Returns service (Royal Mail) and the Euro Returns service (PFW) and that label doesn't look like either. I could be wrong about, but I'm fairly sure. The former service has a weight limit of 2Kg and maximum length of 60cm, the other one will take a higher weight but will be limited to 150cm.
Did you buy it off the Italian arm of Amazon? If not, you might want to have a word with their customer services.