Introducing customs declarations after Brexit could cost traders £4 billion according to this report from the Institute for Government: Implementing Brexit: Customs
The authors say that while most people recognise the customs 'cliff edge' in the UK, not enough attention is paid to a similar cliff edge on the other side of the English Channel. Unless Calais, Dunkirk, Rotterdam and other European ports are also ready for Brexit, British exporters will face significant disruption to their supply chains. Preparation on both sides is particularly vital in the case of the Irish land border.
My preference would be to remain in the customs union or at least have a customs agreement with the EU, but the Government seems to be veering in the direction of a harder Brexit.