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.
Synonym Discussion of sunder. separate, part, divide, sever, sunder, divorce mean to become or cause to become disunited or disjointed. separate may imply any of several causes such as dispersion, removal of one from others, or presence of an intervening thing.