I wish to register a complaint

I wish to register a complaint about a letter that seems to have been opened somewhere along the way but apparently, it is not possible to send the Royal Mail any email.

It's not like I want a huge compensation because the missing contents of the envelope are hardly worth the office clerk's pay for the time it would take them to process the claim, I just wanted to let them know I'm not too happy about the incident.

I did report it to the Finnish postal service, though, because it is obviously also possible that they are to blame and not the Royal Mail.