After poking and prodding for hours, I’ve figured out the fix for this.
In models/Cart66Cart.php before the line “cart66_after_add_to_cart” add “Cart66Session::set(‘Cart66LastPage’, $_SERVER[‘HTTP_REFERER’]);” (without quotes, of course).
What I discovered was that the last page visited variable (Cart66LastPage) is only set in two places in the code. In views/cart.php and models/Cart66ShortcodeManager.php. The first file only checks if the session variable is empty, and if so, sets it to either the store page from the settings or the blog URL. The second file only sets the variable if Gravity is used, which is only available in the Pro version.
Basically, the developer apparently really wants us to buy the Pro version =p