Can´t Update an Invoice

I trie too update an invoice but i can´t.

My sintaxis is this

$strjson = '{
   "customer_id":"xxxxxx",
   "invoice_number":"xxxxxx",
   "custom_fields": {
       "index": "1",
       "show_on_pdf": "true",
       "value": "autorizado",
       "label": "G-FACE"
   }
}';
$url = " https://books.zoho.com/api/v3/invoices/ xxxxxx ?authtoken=xxxxxx&organization_id=xxxxxx&JSONString=" . $strjson ;

Then i call de URL with httpful (work fine when i get an invoice), this is the URL

https://books.zoho.com/api/v3/invoices/xxxxxx?authtoken=xxxxxx&organization_id=xxxxxx&JSONString="{ "customer_id":"xxxxxx", "invoice_number":"xxxxxx", "custom_fields": { "index": "1", "show_on_pdf": "true", "value": "autorizado", "label": "G-FACE" } }"

Please help me with that