For Canadian IT staffing agencies working corporation to corporation: from the job posting and the RTR to the signed SOW, the approved week and both invoices.
RTR before every submission
A candidate goes to a client only with a signed right to represent, and never twice for the same job.
Two invoices from every approved week
The client approves the hours; the contractor's invoice and the client's invoice are issued together, then frozen.
Tax worked out from the place of supply
GST, HST, QST and PST by province, from dated rates your accountant can check. A calculator, not tax advice.