Unfocused inputs in a touch screen interface

On a touch screen cash register, the user has an always visible onscreen touch pad to enter currency amounts. In some cases the register may show more than two fields, but here is a screen shot of our current solution in a situation where only "Amount" and "Tip" may be entered:

As you can see, the focused field is dark and bold, and the unfocused field is grayed out, and has a note to indicate that a tap will focus it (this screenshot was taken on a desktop, but on the touch devices where it's typically used the text reads "(Tap to Edit)")

Tapping toggles the focus:

While this interface works well enough in the field, there is something that strikes me as clunky about having to explicitly say "(Tap to Edit)" to ensure that users know the input focus may be changed. I feel that with a better design it would somehow be obvious.

Is there a way to improve this?

NOTE: Using the native keyboard (ie, one that isnt always visible) was tested in the field and failed: switching focus was too slow for the fast-paced register environment, and the buttons weren't big enough and caused mistypes. So that is not an option.