So if I change the value of the gift I want to send at the gift center each time I use it for Mysterious Melody, I don't have a problem.

But if I want to use 100,000 each time, it'll occasionally (repeatedly) give me the error message that I must wait a few seconds. I waited 15 minutes or so (long enough at least to go grab food) and still got the error, until I went and changed the amount.

After changing the amount, it hasn't given me the error (yet) but I wanted to point it out because it was really annoying.