Paypal _cart not showing tax and shipping in sandbox - paypal

Question

I've got a paypal form to request a payment that I'm testing in www.sandbox.paypal.com.
It works as I expect, except for tax and shipping costs, that are ignored and not shown in paypal cart and not computed in cart total amount.
In this case I only have one product, but buyers can add more products to their carts.
I can't understand where I'm going wrong.
<form method="post" name="paypal_form" action="https://www.sandbox.paypal.com/cgi-bin/webscr">
<input type='hidden' name='cmd' value='_cart'>
<input type='hidden' name='business' value='myEmail'>
<input type='hidden' name='charset' value='utf-8'>
<input type='hidden' name='currency_code' value='EUR'>
<input type='hidden' name='notify_url' value='http://mysite/shop/ipn_url.php'>
<input type='hidden' name='custom' value='xxxaasxdfd'>
<input type='hidden' name='return' value='http://mysite/shop/confirm_url.php'>
<input type='hidden' name='lc' value='IT'>
<input type='hidden' name='email' value='buyer#mysite'>
<input type='hidden' name='first_name' value='Test name'>
<input type='hidden' name='last_name' value='Test surname'>
<input type='hidden' name='zip' value='00000'>
<input type='hidden' name='no_note' value='1'>
<input type='hidden' name='cbt' value='Return to mysite'>
<input type='hidden' name='rm' value='2'>
<input type='hidden' name='cancel_return' value='http://mysite/shop/cancel_url.php'>
<input type='hidden' name='upload' value='1'><input type='hidden' name='amount_1' value='129.00'>
<input type='hidden' name='item_name_1' value='23-PROD11'>
<input type='hidden' name='item_number_1' value='120'>
<input type='hidden' name='tax' value='39.37'>
<input type='hidden' name='shipping' id='shipping' value='67.87'>
</form>

Answer 1

I found that the VAT and the shipping cost are visible and computed if I pass them for every single item in my cart.
<input type='hidden' name='shipping_1' id='shipping' value='67.87'>
<input type='hidden' name='tax_1' value='39.37'>
I've charged them on the first item as shipping_1 and tax_1 in the cart for all my items, in this way I'm sure that it will work for cart with one or more items.