If you send parcels to consumers in Northern Ireland using a carrier authorised under the UK Carrier Scheme, there are no customs declarations and no customs duty, and you do not need to register for any scheme.
For parcels to businesses, your carrier can move them without a full customs declaration or duty if the sender or the receiver, whichever acts as importer of record, is authorised under the UK Internal Market Scheme and the goods are for end consumers in the UK. Otherwise a full customs declaration is needed. Ask your carrier how it handles business parcels.
Send parcels from a business in Great Britain to Northern Ireland (opens in a new tab)
Consumer and business parcels, the UK Carrier Scheme and when a declaration is needed.