In my experience IMSTO is slow but reliable. I get the sense that he is self-conscious about his English ability, which makes him less verbose with his communication (or not at all). He's sometimes hampered by slow and unreliable suppliers.
Him having a store on the web makes it seem like it works like a normal store. You pick, you pay, they confirm, they ship. But the process isn't usually that smooth.
I understand your frustration; I think contacting him will work but it may take a little while for him to get back to you.
Good luck!