Letter sent on Dec 15 still not arrived

Hello, not really sure what I should do at this point. This is the sort of letter with a code I need to activate, and it was mailed Dec 15 but has yet to arrive. However it is a letter from the states so maybe there’s a further delay? I got contacted by the mailer about it but still no letter. not sure what to do/if I should just request a new one.

Any advice would be greatly appreciated :)