Which is why you should take the steps outlined above to insure that you've found the right vendor for you. The trust factor comes into play extensively during this stage. One more note on non-refundable deposits. They are there for a reason. A non-refundable deposit is one of the best ways to do this.
Most vendors are very open to negotiation when the subject of fees come up.
Of course there are some items that have a set price due to cost of materials, time, etc. It's easier on everyone involved if you have a good solid reason to ask for a fee reduction. Any changes of course should be added to the contract. Be honest with your vendor. This is perfectly acceptable as long as both parties are happy.