Postie stealing items from my parcels, re-sealing and then delivering

So my normal postie is a top man a great friendly guy who will go on it his way to please, Everything always gets delivered. There is however a postman who will fill in when my postie is on holiday 80% hours of the time. I Order medical cannabis and it’s delivered via Royal Mail tracking. Whenever this temp is covering the day package has been opened and around half is taken out. This has now happened 3 times and I’ve just been told there are a number of others in the village that have had stuff go missing from parcels. I’ve reported to the Police and they are not interested, I’ve raised complaints with RM and they are looking into it but that was 8 weeks ago. Luckily the company that I use have switched to DHL but it’s not right and he’s been getting away with it for months now, what else can I do? I’m at work when he delivers normally.