Hi,
Your required app fits exactly my expertise: BlackBerry 10 and Android (also BlackBerry OS4.5+). So, let me get this app complete successfully for you. I am looking forward for a long-term collaboration if you want to.
Usually, I will request about 20% of the project price to be made before I start it. I am fine that you keep 20% of the money (~$55) until the app has been on the store for 30 days because this is a small app, it should not have bugs which can't be identify when we test it.
cheers,
mark