Under the new arrangements, the only tariff that could conceivably be levied in relation to goods in Northern Ireland would be in relation to those that come from GB where the idea is to circumvent the rules of the single market and the ultimate destination is mainland Europe. Even then the UK has a double safeguard of waiver or reimbursement. That is not something that was there before.
In terms of the regulatory side of customs, the paperwork of customs, there will be none from Northern Ireland to Britain. Great Britain to Northern Ireland already has regulatory declarations, there is already a regulatory border but not a customs border. So there will be an incremental additional section that would have to be filled out on the form. But that is not a substantial extra burden for business.
And, overall, what Northern Ireland businesses get is remaining part of the UK customs territory, no infrastructure at the border with the Republic and frictionless access to the single market. It is a cracking deal for Northern Ireland businesses.