Do I need a customs label when posting share certificates internationally (UK to USA), and if so what should I write on it?

I live in the UK but have some old share certificates registered with the company registrar in the US (the EQShareownerOnline site).

I want to convert them to DRS (Direct Registration System) registration, and to do this I need to send the share certificates in (more info on this under the DRS and transfer topics in the FAQ at https://www.shareowneronline.com/help/ ).

The thing I am unclear about is the details of posting some potentially valuable paperwork internationally:

• Do I need to fill out a customs label? The Post Office’s information at https://www.postoffice.co.uk/mail/customs-forms says "Letters and large letters containing only correspondence, commercial invoices or shipping documents do not require a customs declaration" but its not clear to me whether this qualifies as simply "correspondence".

• While the value of the shares represented by the certificates is several thousand dollars, the "replacement cost " EQShareownerOnline recommends to insure them for (4%) is conveniently just below the £250 limit that RoyalMail’s "International Tracked" covers. That might also mean that, if a customs label is required, the CN22 one can be used ("items worth up to £270"). But if one is required, what should I actually write on it? "Documents with a replacement value of £250" or something like that perhaps?

Here’s the CN22 form, just for completeness: