The UK Government has said businesses in Great Britain should still be sending parcels to Northern Ireland amid concerns over cancelled orders and delayed deliveries.
Customers of firms like John Lewis, Next and Dunelm have all been told their deliveries are delayed, while orders with TK Maxx are cancelled.
Under the NI Protocol, companies sending parcels here have to complete customs declarations.
While a three-month grace period is in force, it was not announced until shortly before the protocol began to apply at 11pm last Thursday.
But freight boss Seamus Leheny of Logistics UK dismissed a call by North Antrim DUP MP Ian Paisley that the protocol should be dropped.
Other problems including snarl-ups at ports in Great Britain for lorries carrying food to Northern Ireland were discussed at a hearing of the NI Affairs Committee yesterday.
Mr Leheny told the committee that one company had 15 loads of food bound for here which could not be lifted by the hauliers because there was no accompanying paperwork. He said a "breakdown in communications" between government agencies and the industry in Great Britain has helped create delays.
Many companies there had wrongly believed the "grace periods" to help the flow of supermarket goods into NI meant no documentation at all was required. However, pre-notifications to the Department of Agriculture, Environment and Rural Affairs (Daera) are still required.
"There is a yawning gap in the knowledge of those in Great Britain exporting to NI," said Mr Leheny.
Mr Paisley said that the complications seen in the first few days of the protocol meant that it should be scrapped.
"A blind man on a galloping horse could have told you this would be a nightmare," he said.
But Mr Leheny said there was no "realistic alternative". "You can't just walk away and think something else will be better."
He added: "Industry here did support the backstop but the protocol is where we ended up because every alternative was rejected. The view of our industry is, let's make it work."
But despite reports of delays, a spokesman for the UK Government told the Belfast Telegraph "the grace periods for supermarkets and their suppliers are working well".
"Lorries are travelling through NI ports and we continue to work closely with traders as they adapt to these changes.
"The overwhelming majority of parcels continue to move smoothly, and there is no reason for businesses not to continue to send such goods to NI."
The Government is understood to see the three-month grace period from official certification for parcels as a "sensible phased solution".
However, operators are being urged to carry out movements "in compliance with full requirements" as soon as they can.
The flow of goods is being monitored by HMRC. A Government source said the priority "is to have a pragmatic approach that allows us to comply with the protocol without causing undue disruption to businesses and citizens".
Crona Clohisey, public policy lead at Chartered Accountants Ireland, urged traders to ensure they have correct documentation, and use the Government's Trader Support Service for help with the system for moving goods from Great Britain to NI.
"The reality is that while the UK and EU have concluded a free trade agreement, the NI Protocol brings a whole host of new trading rules to crucially enable frictionless trading on the island of Ireland," she said.
"Customs declarations are now required when goods are moved from Great Britain to Northern Ireland because NI must comply with EU customs and VAT rules at its ports."