I.... agree :-)

Exactly as You mentioned - if the validation is part of the Amount value object, we can't construct the Amount without this validation, and the code of Item will be simpler + safe - we can't forget validate input.

Plus in the future we may start offering amount of products in decimals (m/kg), and in such case we'll implement this feature only in the value object itself.

This story + model is made, so I could change the term Amount to Quantity. But if the term Amount comes from the Domain, we mustn't use different term and we have to stick to the domain (and maybe less accurate) Amount.