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.
The Curious Case of the Word 'Sonder' The word was created by a blog on Tumblr very recently, yes. It cannot be found in any 'official' dictionary, except in some other languages such as Afrikaans and French where it has different meanings. The origin of sonder is most likely not related to any word and just made up.