Brexit discussion thread XIV (Please read OP before posting)

The customs union allows trade in goods between its members, free of customs duties. Trade in services doesn't attract customs duties, so the question of freeing trade in services from customs duties doesn't arise.

The customs union doesn't facilitate trade in goods in other ways. Within the single market, goods coming from (say) France to (say) Ireland are presumed to comply with applicable goods standards, because common standards for goods apply throughout the SM. This is an aspect of the SM, not of the customs union, so a territory that was in the customs union but not the single market wouldn't benefit from this. Goods imported from such a territory would still have to be demonstrated to be compliant with EU market standards, and there would be cost/delay associated with satisfying this requirement. This is what's called a non-tariff barrier to trade.