I have a single use certificate but it has exempted a second invoice. If it's a single use doesn't that mean it only works once?
The term " single use" might be a little misleading. What it means is that it is to be used against a single job, which is why there has to be a PO number for the certificate to be effective. You can use that same certificate against multiple invoices for the same PO number, as it is the PO number that carries the exemption. The single use certificate will not work without a PO number.