Seems like a conundrum to me.  Have you asked if you can purchase a primered frame?
If not;
Ask if they are powder coating their frames. 
Then find out if they use a water base paint, or solvent based paint. (Can make a difference in what paint you use and your preparation.)
If the frame is clear coated over their color paint, then at a minimum, the clear coat will have to be sanded, and perhaps sealed, before you paint the top coat you want.
If you receive a bare frame, then you will likely have to fill and sand any defects on the frame, before the primer, color layer, and perhaps clear coat.
(The other thing I don't like is that if you paint over their paint, you are adding more weight to the frame.  Not much, but it adds up.)

On a side note, Automotive/high quality paints have become so expensive here in the states, ($50+/qt) that I would consider paying the price that the frame seller charges to custom paint your frame.  May be worth it in the long run.

standard matte : free
standard matte is between black matte and UD matte , It have clear coat protect the paint
 
UD matte , UD glossy : $105
One color: US:$70.00(matte or glossy,solid color )
Two colors: US$ :95.00 ( all matte or all glossy,solid color )
Two colors: US$ :105.00 ( matte and glossy,solid color )
Two color gradient : US$ :115.00 ( all matte or all glossy ,solid color )