As a rule, non-EU sellers register for IOSS through an intermediary established in the EU. The choice of intermediary and the allocation of liability should be made clear by contract.
Two points shape the setup. IOSS covers consignments with an intrinsic value up to €150; above that, ordinary import VAT applies. And the split of responsibility between you and the EU-established intermediary belongs in the contract, settled before you appoint them — along with a check on the intermediary’s standing.
Shall we apply this matter to your situation?
Tell us your specific situation in a few sentences; we'll assess it with the right team.