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.
If you send me an email, I will write you back. It may take me a week, or a year, or five years, but I value tremendously what random strangers have to say, and tell me about what is going on in their heads. Nothing is more rare or valuable. After all, it's a peek into the heart of the protagonist. That, to me, is the meaning of sonder.