They may be asking for your EORI (Economic Operators Registration and Identification) number. An EORI number is mandatory for customs clearance in the customs territory of the European Union. An EORI number means that you are known to HMRC and can be invoiced for the VAT and Duty after the parcel has been delivered.
I suggest you message them and ask the question. If you don't have an EORI number, they may not fulfil the order because so many customers refuse to pay the VAT and, if liable, Duty, and the parcel is returned to sender - at the sender's expense.
