The second one is charging hourly. We tend to prefer hourly fees on projects that are particularly open ended or very small scope. Our hourly rates are currently between $60 and $120 per hour depending on who is doing the work.
The word sonder exists, though. It means to probe, and in a more specific use, to test the depth of something. The word sound (as in the water feature) is a cognate, both related to the Old English sund meaning to swim, or the sea. I can't really think of an English equivalent that's specific beyond mere awareness.
Definition of tinder. 1  1 : a very flammable substance adaptable for use as kindling. 2  2 : something that serves to incite or inflame that rhetoric was ready tinder for revolution â Margaret Peters.