In February on our trip to China, I had bought some stamps in the hotel gift shop in Shanghai, put them on the postcards and gave them to the front desk of the hotel to mail. My friend gave them $$ and her postcards to mail. Those postcards never made it to their destinations (and unfortunately, those were the ones to my grandkids). I don't know if the person at the desk pocketed her money or what. Mine had stamps on them, but we turned them in together to the desk, so maybe they threw them all away or else put them in a drawer and forgot. I wasn't happy, but what do you do? Too late. You are already back home by the time you figure it out.
However, in the hotel at Xian or maybe it was Beijing, we did the same scenario (I had put stamps on my postcards, my friend gave them $$ to buy stamps), and all of those cards made it to their destinations. I think it was thievery or incompetence in the first hotel.
I will always follow the "put my own stamps on the postcards" scenario, and if possible, if close enough and safe enough to walk, I will mail them at a post office or postal service place. In all honesty, I had never had trouble mailing postcards from hotels before, but this incident in China soured me on doing that again, if I can at all avoid it. My grandkids were disappointed. They save all of the postcards that I send them from trips.