IOSS -VAT for EU sales post office

Hey everyone so just getting to terms with these new EU Vat rules, we barely sell any product to EU customers however today we have 2 orders, it’s showing the ioss number on the shipping page, but the problem is I manually write the addresses onto the parcels and pay directly at the post office, so where does this leave me? Do I just write the ioss number below the address or on the cn22 label at the back?