Whatever deal Britain gets, it will almost certainly not be in the EU customs union, as that mean letting the EU make trade deals, and one of the arguments for leave was that the UK would be able to make its own trade deals around the world.
As such, orders from the uk to the EU will be subject to VAT if valued over a countries threshold (20ish euro?) and duty over a higher limit, plus handling fee. Most small uk businesses are not VAT registered. That will be an option so VAT would then not be (sort of) charged twice, but I'd imagine it will still be off putting to EU customers used to no additional charges.
It's going to be hard for us wee mini companies. But we're not there yet so keep on buying!