How about an itemized list of work to be done with corresponding list of materials and total cost for each item.
I had a local contractor do some kitchen remodeling for me. Had excellent references and lived in the neighborhood. He had about 10 separate items to be done, but gave me a price for the entire job. Work was excellent, but he got held-up finishing the job due to some product not being available (that I was to provide), he said that he would have to move on to a big job, but would come back in the next week or so, and finish-up..."about a day-plus to complete." Said, he would cost out the remaining work,and I could pay him for the work done, less the work pending.
He came up with $300 for the pending work...I paid him, less this amount. Needless to say, it has been a month now, and he hasn't come back. On checking with other contractors to get the pending work done, as a contingency, they almost laugh at the $300...closer to $1,000. My faith still remains the work will be done, because he left his tools for the work, and I have been told being "timely" is not one of his assets. But, as one individual told me, $300 is pocket change with the demands for good contractors today...once they move on to another job...they would probably welcome you telling them to come get their tools and take a hike...
My point is that, even though the fault of not having the product was mine, and I could understand he had to move on to another job, rather than wait...I should have held back sufficient funds to have another contractor complete the job, or at least what he stated in an itemized bid before he started...not what he said the cost was in the above scenario, at the end of the job.