I bought the mailbox. I had 170ish gems, so after my purchase I had 150ish.
I placed the mailbox and got my reward of ten gems.
Then the game reloaded and my mailbox was gone. My gems are also gone - I'm back down to 150ish.
The mailbox is not in my cellar.
My goal to buy the mailbox is also gone. So if I buy it again I won't get the rebate.