That was DS's experience also. The WP PX shoe department ladies were so helpful and patient because he had to keep asking for smaller and smaller sizes. He went from a "tennis shoe" 12 down to a "recruit boot" 10. If you have to buy over the phone, go to a shoe store and get your foot measured properly. It would be best if you could try them on!
If issued on R-Day, I'm assuming they know your shoe size already.
The 340s are not authorized. Those are flight/tank certified boots. Get the 390s.