Some of our UK customers insist we use a particular customs agent for their goods, for delivering into Switzerland.
As a company we normally use one of two agents who know our preferences after we’ve been their customers for many years. When we see some of the other agents names on our job sheet, we know it’s going to be a long, slow process for the clearance 
It’s much the same into Germany now. We have worked with several of the big logistics companies for many years, they now also operate as customs agents for UK goods. They have probably been doing it for many years for imports and exports from none EU countries anyway, so knew what needed doing.
The first few weeks were a bit problematic, but now it’s usually a fairly smooth process.