Post Brexit (01/Jan/2021) postage issues between EU and UK

This has been discussed a bit in the Brexit thread already, but maybe useful to discuss specifically for buyers and sellers on here.

For me personally, posting France to UK:
I took a boxed up frameset into the local post office in France. I had the usual international postage doc already filled out (sender name/address, recipient name/address, desciption of item, value of item, date, signature). I was told that as of 01/Jan/2021 I would now need to download and print out additional customs forms, and bring them with me (preferably already completed), for all packages to the UK.

Details from La Poste on the new requirements (in French):
https://aide.laposte.fr/contenu/brexit-au-1er-janvier-2021-les-nouvelles-formalites-douanieres-vers-le-royaume-uni

English translation:
"From 1 January 2021, trade with the United Kingdom is no longer considered as intra-European trade, but as trade outside the EU.
As a result, new customs formalities, as well as tariff supplements apply to shipments posted after January 1.

You will need to include the following with any shipment of goods to the UK:

• Two copies of the CN23 customs declaration. (Note: This CN23 declaration must be made electronically, and before shipment of the goods. You will be guided step by step through this process, whether you do it online or at a post office.)

• The proforma invoice for a non-commercial shipment, or the commercial invoice for a commercial shipment. (Note: The UK requires these documents to be completed in English)

These formalities are necessary for any package containing anything other than documents (documents on their own are exempt from declaration), and whatever the nature of the shipment (including whether the goods are sent free of charge, as a gift, or a sample), and they are required whether it is a personal or a professional shipment."

With regards to the CN23 customs declaration itself:

"To create your CN23, you will be asked for:

• The nature of the shipment: non-commercial (gift or sample), sale, or return.
  
• The content of the shipment: the nature of the various items, their weight, their value, their tariff number, and their country of origin.
  
• The sender and the recipient (with their respective phone numbers).
  
  

The CN23 must be completed in French, English or in a language accepted by the country of destination.

Print 3 copies of the CN23 (two to attach with the shipment, one to keep for yourself).

How to attach customs documents to the shipment:

• Place all documents in a transparent plastic sleeve.
  
• Stick this pocket on the packaging, preferably on the same side as the shipping label (Note: do not stick the CN23 and invoices directly on the packaging, as customs services must be able to remove them.)
  
• Keep the 3rd copy of the CN23."
  
  

I haven't got as far as finding out how much the price has gone up by yet.