As F-F indicates, there's no point in contacting the Post Office. It's a completely separate company to Parcelforce Worldwide (who have the HMRC contract to inspect postal imports) and their sister company, Royal Mail, that's got the responsibility for delivering the mail to you and collecting the relevant charges.
Try using this web form
https://www.parcelforce.com/help-form
or phone Parcelforce Worldwide on 0344 800 4466
or Royal Mail on 0345 774 0740
(Note though that both organisations are currently running with fewer staff than usual in their call centres, so you might have a long wait).