YUP! I'd stick with the advise given by jperkins. I remember literally pulling out all the hair on my head when it came to paying the bill. I had to convince the customer the number of times the idiot made me change the features because of his mistakes in the original drawings!
If there are plenty of drawings, then you can go through all the drawings and fix a flat rate for all (this is possible only if they aren't too complicated). Otherwise, the per hour rate is the one I'd like to suggest you myself.
I have been doing consultant (or freelance) work for years in Pro-E and 3 other major CAD packages. Each company usually requires a bit different approach depending on their needs, but MAKE sure they understand everything up FRONT in writing as to how you will charge them. I go by the job, the hour and by per model, but track time and changes well so there are no questions on the invoice. It is easy to get caught in that being nice trap and regretting it later. Be fair, but firm. R&D work especially can really bite you big in hours if you are not careful.