wisex Sorunu çift where kullanarak çözdüm.
$toplamTutar = Product::where('name', 'like', '%' .'vekalet'. '%')->where('invoice_id', '=', $invoice->id)->sum('amount');
$toplamAvans = Product::where('name', 'like', '%' .'avans'. '%')->where('invoice_id', '=', $invoice->id)->sum('amount');