But as of next March, when Britain is supposed to bid adieu in a process known as Brexit, the Netherlands and the United Kingdom are set to become separate markets , divided by a newly important border.

Unless the two bickering parties in this messy divorce reach an agreement bringing about some less disruptive arrangement, goods traveling across the channel are likely to confront customs checks and sanitary inspections. Flowers now shipped duty-free may encounter tariffs.

The word on Wednesday that the British government had struck an agreement in principle with European counterparts on how to proceed merely moved the conflict into the British Parliament, where the deal could be voted down or altered. The governments of the 27 remaining nations in the European bloc must bless the deal, an uncertain proposition. For people who sell goods across the channel, clarity remained distant.

“At both sides, the governments are very willing to prepare, but still you need hundreds of people, and you need to train them, and you need warehouses to do the checks, and you have to build the computer systems,” Mr. Pasma says. “There are going to be major traffic jams in the Netherlands. If it happens tomorrow, it’s going to be a huge problem.”