I have a few questions. forgive me if any one asked these things. I didn't read all the posts.
you say you removed 3 truck loads of mulch. What happened to removing one trailer load?
You put down 3 truck loads of top soil . same question as above?
The charge for the fuel may have been 0nly $60 but you made a lot of unnecessary trips and took a lot of time that she shouldn't have had to pay for.
When i run equipment I expect to get payed for running equipment. equipment has to pay for itself. Now wether or not you needed a tiller for such a small area is another question.
I believe in the charge as much as you can system. so I think your price is fine from my point of view. If I was the customer I would be wondering why I was paying $40 an hour for some one to drive his truck around. I know you are just starting and don't have all the necessary equipment but that isn't something the customer should have to pay for.
You didn't over bill. Where I come from we call that milking it. (said with a smile)