Deutsche Post steals the necklaces from the envelope

Hi everyone, I have been selling my old beaded necklaces on Vinted Germany. I put them on a plastic bag and then I also put a postcard in there. However, still the delivery people can tell it’s a necklace and they steal it. So far three times the side of the envelope was ripped and necklace was taken. I have changed my Deutsche Post shop and all three arrived to different cities. Why is this happening? How come Germany and Deutsche Post is not safe enough? Are there any recommendations to prevent this?