My Checkout is directing me to a 404 page. Please help!

The checkout link for my store is not adding “/index.php/” to the link and is causing a 404 error. All the other links that I have on Marketpress are working fine until you start the checkout process and that’s where the links are not correct.

The link currently is “http://proav.celabs.net/buy-online/shopping-cart/”.

Where it should be “http://proav.celabs.net/index.php/buy-online/shopping-cart/” which actually works and continues the checkout process.

I have my permalink settings correctly set to “http://proav.celabs.net/index.php/sample-post/” as indicated in your walkthrough videos.

Could you please tell me what I need to do in order to straighten this issue out?