Our company self certifies UL 508. After initial fees and testing, we are able to put the stickers in our machine's panels. We started that in the 70's, so I'm not sure what the initial procedures were, but I know we had to submit lists of parts.
Currently we pay an annual fee (based on how many files you hold). Then we pay a fee every time the inspector shows up for the quarterly inspection. He checks to make sure all of your components have the UL mark on them and charges you several hundred dollars for it.
To add insult to injury, we pay a fee for the stickers themselves. Apparently, charging over $100 for stickers wasn't enough, they decided to add a new $25 "handling fee".
Personally, I feel it's a racket. However, it is unavoidable, since most (if not all) electrical inspectors in the States require it. The sad part is, UL 508 code just reiterates what is already in the NEC code book. All of your parts must be UL certified or UL recognized. Which in turn charged that manufacturer for having a UL mark.