How can I set up the MarketPress checkout screen so everything lines up on one column?

How can I set up or style the MarketPress checkout screen so everything lines up on one column when it is viewed on a 320 pixel wide screen? Right now it looks terrible on the checkout section because it is trying to fit two or three columns into only 300 pixels.

Also because of the "State/Province/Region" dropdown it is messing up my responsive layout at this size because at the bottom of the list this item is taking up too much space: "AE - Armed Forces Africa, Canada, Europe, Middle East". It would be great if that one option could either be turned off or broken into two lines.

How do I adjust these things? Thanks.