My opinion, mj pay itself is based on money laundering (taking money from illeagal means and placing then into the banking system). Eveyone with the app on their phone will be in the feds radar with paper trail.
When pot becomes leagal, we don't need the app as we can use Google wallet, apple pay, debit, credit, cash, checks, all which are universally used. Not a sufficient means for a successful business model.