How do you phrase that exactly? Move the collar buttons 1 cm further away from the placket? Is that right?
I try to explain as thoroughly as I can. Usually something like this: Please move the buttons in the button-down collar 1cm more apart (towards the sleeves, away from the center). What you said would work. While they are based in China, their customer support are native English speakers, so there shouldn't be any problems communicating with them.
However, if I were you, I'd order one default shirt and see if you yourself want to move those buttons. It's very personal and different people want the collar to fold differently. It's also easy to move them yourself. Oh, and make sure you specify the collar to be SOFT, otherwise it will be too stiff.