Wasn't the time spent to write this post worth more than 1.5k the OP is whining about?
While I agree that doing work for free can be a trap, I still believe that the one metric the OP dismisses (time spent adjusting the bill vs time spent just implementing the feature) is the most important: It might feel "good" to bill for every second but overall the benefit of being known as the "good guy" always outweighed the cost for me. I even once made the incredibly stupid mistake of billing (in the sum) for one more hour than I worked for them. I pointed that out and the response was: don't worry, just deduct it from the next bill - and even if you don't, you saved us so much money that we'll find a way to make up for the difference by creative book-keeping.
I think it is all about managing expectations. Don't do work for free unless you want to, and let the clients know you are doing it for free (and thus it comes without the guarantees of paid work).
While I agree that doing work for free can be a trap, I still believe that the one metric the OP dismisses (time spent adjusting the bill vs time spent just implementing the feature) is the most important: It might feel "good" to bill for every second but overall the benefit of being known as the "good guy" always outweighed the cost for me. I even once made the incredibly stupid mistake of billing (in the sum) for one more hour than I worked for them. I pointed that out and the response was: don't worry, just deduct it from the next bill - and even if you don't, you saved us so much money that we'll find a way to make up for the difference by creative book-keeping.