The short version: most holds are documentation queries, most resolve, and the paperwork description is the variable that predicts them.
Personal importation rules differ from commercial importation rules in most jurisdictions, and consignment size and packaging are among the factors that determine which applies.
In practice, a seizure notice, where one is issued, is a formal document with a stated basis and a response deadline. It is worth reading carefully and, if the amount matters, worth advice.
Seizure procedures include a stated basis and a defined response period in most regimes, which is why the notice itself is the important document.
Prior successful deliveries establish nothing about the legality of the next one.
Keep every document. It is the only record that will exist.