This is an absolute minefield
"In order to prepare for Electronic Customs data becoming mandatory, we will be updating Click & Drop to allow electronic customs data on orders for all international destinations, including Europe.
Electronic customs data can be added to your products, imported along with your orders via spreadsheet, or added when applying postage to your orders.
After the update, if you have the ‘Generate customs declarations with orders’ label format setting checked, then CN22/CN23 documents will be generated for all international orders."
For more information on how to enter electronic customs data within Click & Drop, please see the following guides:
https://help.parcel.royalmail.com/hc...ational-orders
https://www.trade-tariff.service.gov.uk/sections