You pay the basic fee (or administrative charge), whether the certification comes back positive or negative or "no opinion". Then with some organizations/companies, you have to pay a % of catalog if positive. Here's the kicker -- certificates have been revoked or reversed among these guys. These are supposed to be the best, but even they will disagree among each other. And don't expect to get your money back.
The rule of thumb I use -- if you aren't sure, sent it to Weiss first. It only costs $10 for the opinion, plus the shipping, and an extra $5 if you want a certificate. His opinions/certificate are widely recognized in the US. Maybe not as glamorous as APS/PF/PSE, but just as good. Since your ID is a color related issue, I don't recommend using the picture/scan service, obviously.
He is VERY VERY good, and first on my list of recommendations. I've read some of his opinions -- trust me, he is VERY VERY good. He won't give you those hand-waved meaningless explanations of negatives or "no opinions". He is also top-rate when it comes to detecting forged/altered stamps (one of my interests). You can't go wrong with Weiss.
Just my "opinion",

k