'Add to Cart' button too far right-aligned. How to correct?

I just installed Marketpress on my WP blog (running Thesis) and am having difficulty with the 'Add to Cart' button. I've tried centering it or making it left-align, but the button continues to show up on the far right side of the page.

I even tried expanding the px of the page and it continues to push out far to the right. Changing the px width didn't help at all.

How do I alleviate this problem? Depending on which is selected, you can only see 'Add' or 'Buy' when the shortcode is inserted at various places in the page.

Ideally, I'd like to see the 'Add to Cart' button centered directly under the picture I choose within the page.

I'm happy to send a screenshot if that would help.

Thanks for any help you can offer -
-Lauren Hill