Then you should look at whether you advertise it as "unlimited". It could look like mis-selling and I think the service looks good so you should possibly look at this

EDIT: I'd like to just say that I've downloaded and am using the app and its lovely. Really well done. Regarding the bandwidth usage, I think you should still be able to come up with something that is representative rather than what you are currently using. Hosting companies have done it for many years, so just come up with a bandwidth figure per Gb that you are happy with