The exit out of the EU means that any transfer of goods will be an issue to custom, so brexit and customs will be mentioned in the same breath. Besides the fact that the brexit will make foreign goods for the English people more expensive, it is also a question of tough compliance. So far the EU agreed on a lot of simplifications so customs will be clear and consistent. You will find the same custom numbers and categories all over the EU. With the brexit the UK will change their number and this will make it all more complicated. Of course, the supplier usually takes care of all this. But as you can imagine, the more work he has the higher will be his invoice.