It’s important to make clear that the phased import approach (above) does not apply to exports i.e. the flow of goods from the UK to the EU, and particularly between the UK and Northern Ireland, the details of which are yet to be finalised.
While imports to the UK will have a grace period, exports, for now, will not.  This means that from 1 January organisations / logistics providers will have to comply with full customs processes for shipments leaving the UK, significantly impacting procedures both internally (for the logistics provider) and externally (for the client / organisation).
For shipments to Northern Ireland from the UK, this could also mean a transit process will need to be followed.