If you are having letters arrived that have been opened, or not arriving at all, you should contact the Royal Mail immediately. There is wide-spread theft in Royal Mail sorting offices, although it is very rarely reported in the media.
One of the most common thefts is money and postal orders stolen from childs birthday cards, as they are easily recognisable. Which is why I never send cards to children addressed to them, in pastel coloured envelopes.
I put the card inside another nondescript envelope, addressed to Mr and Mrs XXXXX.
Contact the manager at your local sorting office straight away Barmaid !