What i Think your doing is dangling a "carrot on a stick". Just the same as many mobile companies offering you free calls or sms. They/you want them comfortable with your product, used to using it. Then the barrier to going past the free quota is a little lower.

I equally like to a point how your tracking it via excel, instead of building a system for hours. Once proven it might aid growth of revenue, then i see the system being built.

Even if the above is not correct for you, it has given me food for thought and helped solidify a few liquid thoughts.