I have the same problem
i have the same issue and I'm not able to get out
I give you here the example of the code. Many "values" are given by PHP variables.
<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="item_name" value="$product_name" />
<input type="hidden" name="item_quantity" value="$quantity" />
<input type="hidden" name="amount" id="item_price" value="$basket">
the value on $basket depends from the choices of the client. Checking the page source once shown in the browser, the value of $basket is correctly shown.
At this point if you click on PayNow, all the data are passaed correctly eccept "$basket". I mean, the page of PayPal shows an empty input field where the client can write what she wants.
Anybody can help me please?