Hey, sorry for the delay.
The problem is the Decimal Separator setting, "Separatore decimale", in the language options. If you clear the contents of that field, it works as expected.
The reason for this is unknown to me, but I know that the point of failure is in vBulletin code, not vBShop code. Unfortunately I won't be able to change vBulletin code to make it compatible with that decimal setting.