I can not directly help, but as mentioned in another thread, although about the cart, there seems to be an error happening with store owners / previous customers but not others who have never visited the site.
By the way your site works fine for me.
Came here to report the same php errors. The above suggestion by @pirulee is the way to go, looks like you have already done this in class-wcoa-admin.php so should be an easy fix.
