I see you are missing my point completely. I guess that's why you keep bringing in all these extraneous points. It might be that my native language is not english, but I hope I can explain it better this time:
1: CMYK means nothing by itself. Working in CMYK will NOT guarantee a better result.
There ya go. I never said that. In fact I stated that, to my knowledge, CAG requests RGB artwork which was checked for problem colors in CMYK.
2: Ask the printshop which color standard their equipment is calibrated to. If they don't know, find a different printshop.
No printer can print all colors you can see in RGB. Notably not the "radiant" blues that people like a lot in their CPO designs. But indeed it would be nice to have some calibration info. More on that in point 3.
3: Unless your monitor is of reasonably high quality and properly calibrated, you will never have a clue whether the print you received was done improperly. Basic calibration does not require a controlled studio, just a little care and knowledge on your part.
That's basically my question. "Did you calibrate your artwork, or did you just create it in RGB colorspace and send it to the printer". Better to understand if I phrase it that way?
4: Look at the picture of the 2 panel prints side by side, you'll see is one is blue and the other is purple. This is not a CMYK conversion issue. But if you re-read my post, you''ll most likely be able to figure out some possible cause to the problem...
True, there might be more issues. One can start assuming one of a gazillion possible reasons for this, but a prominent one would be calibration problems. Or maybe the printer attempted to correct a problem about the washed out colors noted by Sasquatch based on the first attempt and corrected the calibration to be more colorful, maybe there are different printers ... etc etc etc
5: If this seems like too much for you to concern yourself with, just pay the $40 and cross your fingers. Just try not to be too hard on the guy when you don't get what you expected. 
I was trying to prevent that yes, but since you came bullying the whole thread to pieces I guess I'm not gonna get an answer no.
Bottom line is:
- A CMYK printer will never be able to reproduce all colors that can be displayed on a RGB monitor
- It's possible that above posters/buyers did not use any form of calibration or color proofing. Since I can't see them mentioning it anywhere at least it seems wise for me to ask if they did.
- If possibe, a conversion to CMYK (or indeed if available an appropriate better colorspace) in a computer program will bring out possible problem areas
Hence, I ask if they have taken this into account. Simple question to see if a very common problem has occurred. I just opted to accompany it with a picture rather then a lecture.