VAT payments and tax return submissions for the Import One-Stop-Shop must be done monthly. EU VAT charges can be paid to the IOSS intermediary who will check that the declaration is correct (with sales records from the relevant EU authorities), which will require an IOSS number in order to use. Final payment for non-EU businesses to the EU tax authorities must be handled by an IOSS intermediary registered in the EU, on behalf of the company.
Please note, the IOSS scheme only allows for output VAT payments and does not allow for any reclaim of input VAT. Separate refunds will be given if VAT is incurred in a country where a seller is without valid registration.