Fatal error: Cannot use string offset as an array in /home3/ggbbs/public_html/wp-content/plugins/mar

This wasn't happening yesterday; now every time I click "Checkout", I get this error.

Also, the shopping cart extends over part of the product page and it looks very unprofessional.

Any hints? website: ggbbs.com -- this is just a test website for now.

Thank you,
Dennis Crow