You never really know how a customer is going to react, even if you give them a fixed, lump sum price. I have had more than one customer claim that I was taking advantage of them when I billed them exactly what I said I would.
On the other hand, I have an account isn't all that big, 1/2 to 3/4 acre, but it has tons of mature trees, including nut trees and a big Sycamore that makes a mess with both bark and huge leaves. Lots of nooks and crannies and different yard areas. Add the hurricane mess to it (mostly sticks/twigs but a few larger branches requiring a little chain saw work) and my bill for the second service came to over $600 with tax on top of a $400 bill for the previous service. I half expected her to freak out, but I got the check yesterday with a note on it. Thank you Darryl!