You do realize since success is random, it could take an infinite number of attempts, so there is *no* way to charge a flat rate that will always make you a profit no matter how many attempts?
Success rate isn't random, it's a certian percentage modified by your companions affection and "natural" crafting bonus. I honestly don't know the rate at the moment, but since I generally craft an augmented piece within 6 tries, (My one odd 15 attempts is by far my worst, and about the 10th augmented thing I've tried for), I will use 1/6 for this example.
If my chance is 1/6, I could charge 12x's the materials cost for one. That gives me 12 tries before I start losing profits.
In 12 tries (1/6 x 12/1 = 12/6 = 2) probability wise I should have made 2. Thus I profit.
So your statement is true there is "No way to charge a flat rate and always
profit" but the probability you would lose money is low, and the amount of customers you will get from decent fees + uncomplicated transactions should make up for any once in awhile losses.
<.< For your flat rate, I'd just do a little research into the probability, then stack it in your favor within reason.