Paypal bug: why does it not pick up the shipping value from the shipping variable?

Why paypal does not pick up the shipping value from these variables form,

<input type="hidden" name="shipping" value="<?php echo number_format($cost_shipping,2); ?>">

or

<input type="hidden" name="shipping_1" value="<?php echo number_format($cost_shipping,2); ?>">

or

<input type="hidden" name="shipping" value="<?php echo number_format($cost_shipping,2); ?>">
<input type="hidden" name="shipping2" value="<?php echo number_format($cost_shipping,2); ?>">开发者_如何学运维;

the postage and handling always show 0.00 on the paypal sandbox unless I use handling_cart variable,

<input type="hidden" name="handling_cart" value="<?php echo number_format($cost_shipping,2); ?>">

Something wrong with the paypal sandbox??