

So, you don't mean that the letters disappeared when you collected them, you mean 30 stored gift cards disappeared when you collected the most recent pile of thank you letters, giving you the newest 5 daily gift cards but taking away the 30 you already had saved.
First, double check to make sure that you didn't accidentally buy a mystery package...check all decor and your farm for anything new. If you can find nothing, if you've not already done so, please place an in-game support ticket so this issue can be looked into.
Thank you for bringing this to attention and, hopefully, this is an isolated incident.