As in, we would usually send clients on out MCC an invoice every month. I don't deal with it personally, but is it just a case of setting up the invoice to sent every month. i cant seem to find any info on any pre requisites for this...
netmeg - fyi - it isn't always the client who has to apply for credit when using invoicing. We pay for ppc on behalf of our clients, so we have an agency line of credit that can be applied to any account falling under the MCC