Para todo aquél que haya llegado a mi post, es así.
Código PHP:
<form name="pagar_con_paypal" method="post" action="https://www.sandbox.paypal.com/cgi-bin/webscr" />
<input type="hidden" name="cmd" value="_cart" />
<input type="hidden" name="upload" value="1">
<input type="hidden" name="business" value="email" />
<?php $num = 1;?>
<?php foreach($this->cart->contents() as $items): ?>
<input type="hidden" name="item_name_<?php echo $num;?>" value="<?php echo $items['name']; ?>" />
<input type="hidden" name="quantity_<?php echo $num;?>" value="<?php echo $items['qty']; ?>" />
<input type="hidden" name="amount_<?php echo $num;?>" value="<?php echo $items['price']; ?>" />
<?php $num = $num + 1;
endforeach;?>
De nada.