Powder Coatings are sound, you should be able to send them stuff. They used to paint Saracen's frames back in the 90's before the brand dived in popularity and they specialise in motorbike stuff.
I've had a dozen or so frames done in the past and only a couple where it wasn't quite perfect (albeit still very good).
As nathan says you have to strip the frame down but they blast all the old paint and stickers off. You do need to blank all threads with sacrificial bolts yourself and it's worthwhile masking faces like inside the seat tube and headtube but as the blasting removes this they will re-mask before coating. on occasions the powder does get onto these surfaces so allow for a bit of remedial work yourself or a trip to a well set up shop to get the tubes reamed if you want it to be a perfect job.
The thing about going in person is the colour choices. They have a waiting room with coated metal disc samples, there are 3 walls of colours to choose from including flat, metallic and pearl finishes.