Hi,
Your situation is little weird (if you didn't touch some PHPs)
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Anyway, here is your patch:
Open these 2 files:
ROOT/templates/<yourjoomlatemplate>/html/com_virtuemart/cart/select_shipment.php
ROOT/templates/<yourjoomlatemplate>/html/com_virtuemart/cart/select_payment.php
In both these PHP files, you will find <form .... > element,
Make their action="" attributes as:
action="/cart"
Something like;
<form method="post" id="userForm" target="_parent" name="chooseShipmentRate" action="/cart" class="form-validate">
This should fix your problem.
Regards.
Forum Support.