404 Page not found

Using Firefox or IE — when a user clicks on Checkout from the Shopping Cart page… we get a 404 Error Page — Page not found. The URL is https://carepointrx.com/online-store//?slug=login.php

For some reason, it works intermittently in Chrome, but in FF or IE it just won’t work. I’m assuming it has something to do with the double forward slashes but I can’t figure out where to fix that issue… Help please!

http://wordpress.org/extend/plugins/wp-online-store/