Hi,
Being from Northern Ireland I can tell you that it can be a pain to get stuff sent to us. I have a couple of suppliers that just stopped selling to NI because of the problems.
So there are two things to think about EORI number which you need to give to your supplier to allow them to send products to us, and then there is a UKIMS no which is to stop you having to pay customs duties (as long as the product isn't at risk of entering the Eurozone)
If your client has an EORI and a UKIMS number, then you don't have any problems, as long as they are not sending it into the South of Ireland.
I have a supplier who marks all of his shipments at a value of less than £135 and there has been no issues. I have talked to loads of companies about this, and I can confidently says that none of them have understood the rules and how to apply them.
The government website explains it all, if you read it, then you can learn everything you need to know, sort off, it seems to assume that you have a background in import/export or are some sort of expert. For example, if I buy goods from a supplier in England and then I 'process' them and sell them into the Eurozone, then there are no customs duties to pay, but try to find a definition of 'process' and you'll get frustrated. I have had definitions from repackaging right up to it being a part of a manufacturing process.
Anyway, sorry for the rant. Get their EORI and UKIMS and it'll be OK. Mind you, they probably don't have them. If they are regular clients tell them to get them, EORI first which is quick and easy, UKIMS next which for me was neither quick nor easy.
If they are not a regular client then do what most people do and put it through as a B2C sale.
Good Luck
Jim