Can't get past checkout.php

On 4/21 I asked what it would take to remove PayPal and incorporate Authorize.net.
I think now I have begun to understand what Ray was telling me.
That said, I have 2 questions.

FIRST:
When I now login, place something in my cart and complete the checkout page, the wheels turn and I'm returned to my cart with "Your cart is empty".
There is no recorded order history for these transactions but I do receive:

Subject: IntuHorses, Ltd Customer Receipt/Purchase Confirmation
and
Subject: Merchant Email Receipt

Both in "Test Mode" and both are complete and indicate that everything was A OK.

SECOND:
Would it be correct to assume that until I get past the first problem I won't get to USPS shipping?

This is the test site
INTUHORSES_dbtest/

Please take a look.

Mick