Trouble with shop page only

As I said before, it seems to be a theme issue.
The “Shop page” you referred is the Woocommerce functionality. May be the theme is not compatible with Woocommerce, and that is why it breaks the “main-content-area”.

Try looking at those Woocommerce documents about theme compatibility:
https://docs.woothemes.com/document/third-party-custom-theme-compatibility/
https://docs.woothemes.com/document/woocommerce-1-6-6-2-0-plugin-and-theme-compatibility/

Or some other resources such as:
https://www.inkthemes.com/woocommerce-theme-compatibility/4/#

Maybe you can fix it yourself, make it compatible.

Btw, if you can’t get any reply from a theme developer, changing the theme with another up-to-date and compatible theme is much more reliable, considering the upcoming chances/updates, etc.