Turnover is down 12% at one of the main retail distributors across the Irish Sea.
AM Nexday blamed uncertainty amongst businesses based in Great Britain following the introduction of the Northern Ireland Protocol.
Senior manager Sarah Hards said her organisation recruited extra staff and incurred additional costs to cope with post-Brexit checks and paperwork.
She added: “There does not seem to be enough rewards for us really here.”
Post-Brexit checks at all Northern Ireland’s ports will resume from Wednesday.
Inspections of animal-based food produce arriving at Belfast and Larne ports were suspended recently amid concerns over the safety of staff.
AM Nexday describes itself as Ireland’s number one retail distributor.
Ms Hards said turnover was down 12% from January 2020 to January this year.
Huge numbers of lorries carry food across the Irish Sea every day.
The business development manager at the company added: “Many of our customers are key and have stayed open during every lockdown.
“Suppliers in Great Britain are fearful.”
She told the Northern Ireland Affairs Committee of MPs that some customers were not completely prepared.
“A lot of people just did not think that this was going to go through, something would pull them back at the last minute,” she said.
“In our business we reached out to all of our customers and said that this is what we need you to do.”
Threatening graffiti expressing opposition to a so-called Irish Sea border had appeared in the Larne area in Co Antrim.
Former Stormont agriculture minister Edwin Poots ordered the suspension of checks shortly before stepping down from the role to receive medical treatment.
His successor Gordon Lyons maintained the position.
The Police Service of Northern Ireland has since said there was no evidence of “credible threats”.
On Tuesday, the Department of Agriculture, Environment and Rural Affairs said checks will resume on a phased basis from Wednesday.
Sinn Fein and the DUP are at loggerheads over whether the withdrawal of staff was justified at all.
Unionists and loyalists are opposed to post-Brexit arrangements, which they believe threaten the integrity of Northern Ireland’s place in the UK.
Ms Hards also raised concern about lack of experience at the Trader Support Service (TSS).
She said: “It is not their fault, they have been brought into this role and have no previous experience in customs or logistics whatsoever, so they sometimes fail to grasp what you are talking about, even after a long conversation.
“It would be good to have someone in TSS with customs experience, with a logistics background.”
She said because the TSS staff are working from home, it means there is a delay if they have to contact more senior staff to find answers and sometimes it is not the answer she really needs.
Declarations should be simplified and the grace period for transit of some goods should be extended beyond April, she said.
She acknowledged that officials from Stormont’s Agriculture Department have been helpful.
Further regulation is due to be published on Thursday.
Ms Hards added: “If they are overly-onerous, I do see retailers and customers trying to source goods from somewhere else, possibly on the island of Ireland.
“If there is so much information required it will put people off bringing freight into Northern Ireland.
“I really hope this is not the case but it is just a wait and see.”
Committee chairman Simon Hoare said the Northern Ireland Protocol was “neither Armageddon nor nirvana”.
Business leaders before the committee said they would like further grace periods of light-touch regulation, since the protocol was only agreed a short time before the end of the original transition period.
The Government has asked the EU for an extension and they are due to meet for talks on Thursday.
Nick McCullough, who manages the Northern Irish arm of logistics firm DFDS, said businesses will struggle to be ready for more change in April.
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