yu cant just change the grand total that is passed.
you need to change all of the prices that are passed:
$WA_PP_ECO_Set_params[0][$nextIndex] = "OrderTotal";
$WA_PP_ECO_Set_params[1][$nextIndex] = "".$ecartNRG->GrandTotal() ."";
$WA_PP_ECO_Do_itemized[0][$nextIndex] = "Amount" . strval($cartIndex+1) . "";
$WA_PP_ECO_Do_itemized[1][$nextIndex] = "".$ecartNRG->DisplayInfo("Price") ."";
$WA_PP_ECO_Set_params[0][$nextIndex] = "ItemTotal";
$WA_PP_ECO_Set_params[1][$nextIndex] = "".$ecartNRG->TotalColumn("TotalPrice") ."";
$nextIndex = count($WA_PP_ECO_Set_params[0]);
$WA_PP_ECO_Set_params[0][$nextIndex] = "TaxTotal";
$WA_PP_ECO_Set_params[1][$nextIndex] = "".$ecartNRG->GetTax() ."";
$nextIndex = count($WA_PP_ECO_Set_params[0]);
$WA_PP_ECO_Set_params[0][$nextIndex] = "ShippingTotal";
$WA_PP_ECO_Set_params[1][$nextIndex] = "".$ecartNRG->GetShipping() ."";
$nextIndex = count($WA_PP_ECO_Set_params[0]);
$WA_PP_ECO_Set_params[0][$nextIndex] = "HandlingTotal";
$WA_PP_ECO_Set_params[1][$nextIndex] = "".$ecartNRG->GetCharges() - $ecartNRG->GetDiscounts() ."";
paypal looks at the grand total, the itemized amount * itemized quantity, the item total, tax total shipping total and handling total and makes sure they all add up correctly.
you will need to do some unsupported custom coding to get this working the way you want it to.