Ok, it was a simple trick to do. For those who might need it:
L_PAYMENTREQUEST_n_NAMEm - "n" is the number of transaction, 0 for 1 single transaction - "m" is the number of the product
$item = array('L_PAYMENTREQUEST_0_NAME0' => 'Test product ', //title of the first product
'L_PAYMENTREQUEST_0_DESC0' => 'Description of my item', //description of the forst product
'L_PAYMENTREQUEST_0_AMT0' => '0.01', //amount first product
'L_PAYMENTREQUEST_0_QTY0' => '1', //qty first product
'L_PAYMENTREQUEST_0_NAME1' => 'Test ', // title of the second product
'L_PAYMENTREQUEST_0_DESC1' => 'Description item',//description of the second product
'L_PAYMENTREQUEST_0_AMT1' => '0.01',//amount second product
'L_PAYMENTREQUEST_0_QTY1' => '1'//qty second product
);
$orderParams = array(
'PAYMENTREQUEST_0_PAYMENTACTION'=>'Sale', //becouse we want to sale something
'PAYMENTREQUEST_0_AMT' => '0.02', //total amount (items amount+shipping..etc)
'PAYMENTREQUEST_0_CURRENCYCODE' => 'USD', //curency code
'PAYMENTREQUEST_0_ITEMAMT' => '0.02', //total amount items, without shipping and other taxes
'PAYMENTREQUEST_0_SHIPPINGAMT' => '0' //the shipping amount, will be 0 coz we sell digital products
);
Above you can see an example for two products. These keys and values will be send to express checkout api in order to deliver the token. The vars will be sent with GET.