As it seems to me developers never worked out properly such paypal _cart function, here just thought to share update with everyone:

Just open your paypal.php file which is located in /included/modules/payment/paypal.php

Find the following pointless code (who want the store name as items on PayPal?):
PHP Code:
if (sizeof($optionsLineItems) > 0) {
      
$optionsLineItems['cmd'] = '_cart';
//      $optionsLineItems['num_cart_items'] = sizeof($order->products);
      
if (isset($optionsLineItems['shipping'])) {
        
$optionsLineItems['shipping_1'] = $optionsLineItems['shipping'];
        unset(
$optionsLineItems['shipping']);
      }
      unset(
$optionsLineItems['subtotal']);
      
// if line-item details couldn't be kept due to calculation mismatches or discounts etc, default to aggregate mode
      
if (!isset($optionsLineItems['item_name_1']) || $optionsLineItems['creditsExist'] == TRUE$optionsLineItems = array();
      
//if ($optionsLineItems['amount'] != $this->transaction_amount) $optionsLineItems = array();
      // debug:
      //ipn_debug_email('Line Item Details (if blank, this means there was a data mismatch or credits applied, and thus bypassed): ' . "\n" . print_r($optionsLineItems, true));
      
unset($optionsLineItems['creditsExist']);
    }
      
    
$optionsAggregate = array(
                   
'cmd' => '_cart',
                   
'item_name' => MODULE_PAYMENT_PAYPAL_PURCHASE_DESCRIPTION_TITLE,
                  
'item_number' => MODULE_PAYMENT_PAYPAL_PURCHASE_DESCRIPTION_ITEMNUM,
                   
'num_cart_items' => sizeof($order->products),
                   
'amount' => round($this->transaction_amount$currencies->get_decimal_places($my_currency)),
                   
'shipping' => $oshipping,
                    ); 
and replace it with the following:
PHP Code:
 $optionsLineItems['cmd'] = '_cart';
      
$optionsLineItems['num_cart_items'] = sizeof($order->products);
      for (
$i=0$n=sizeof($order->products); $i<$n$i++) {
          
$item $i+1;
          
$tax_value = ($order->products[$i]['tax'] / 100) * $order->products[$i]['final_price'];
          
$optionsLineItems['item_name_' $item] = $order->products[$i]['name'];
          
$optionsLineItems['item_number_' $item] = $order->products[$i]['model'];
          
$optionsLineItems['quantity_' $item] = $order->products[$i]['qty'];          
          
$optionsLineItems['amount_' $item] = number_format($order->products[$i]['final_price'] * $currencies->get_value($my_currency), $currencies->get_decimal_places($my_currency));
          
$optionsLineItems['tax_' $item] = number_format($tax_value $currencies->get_value($my_currency), $currencies->get_decimal_places($my_currency));
      }
      
$optionsLineItems['handling_cart'] = number_format($order->info['shipping_cost'] * $currencies->get_value($my_currency), $currencies->get_decimal_places($my_currency)) - $shipping_added
the upgrade is complete!