Creat new billing address goes to product screen

It must be because you have a javascript error on the checkout page: www.hikashop.com/support/documentation/6...ty-issues.html#modal

If you provide a URL to your shop we should be able to confirm that.

The problem is something else.
Your template actually has a strange behaviour and display the module in the component.php file instead of the content of the page with the itemid is 3.
I don't know why but the solution seems to be simple.
The link is actually: www.brytongps.com/index.php?option=com_h...l=component&Itemid=3
but if you change the itemid to 2 for example it works: www.brytongps.com/index.php?option=com_h...l=component&Itemid=2
So you can just change the "force a menu on the checkout to the menu with the id 2 and that should solve the problem.