Can you shed some light on what your fees are (if any, and if not, how the app makes money)? This is a great concept, and it's done in the stock market as well (i.e., automatically buying stocks with excess change).
So Lawnmower doesn't actually charge any fees, but all bitcoin purchases are initiated within your Coinbase account (which currently charges $.15 for transactions < $15 and 1% for transactions > $15.) We're exploring revenue model options but our core service will always be free.
To mitigate these fees, we set up a customizable minimum purchase threshold that the user can set. The spare change is essentially accumulated until it reaches this threshold and then a bitcoin purchase for the total spare change amount is initiated for you via Coinbase and sent to a "Lawnmower Wallet" we create.
Appreciate the comment, and yup it's a similar base product/strategy to something like Bank of America's "Keep the Change" program (to save via a savings acct) or Acorns (to save via mutual funds).
