I usually work to an agreed cost for all white label services undertaken, whether that’s project or task based.
For new agency clients, a deposit payment is usually required for the agreed work and the final payment would be settled upon delivery and acceptance of the work.
For larger projects and established agencies, invoices can be issued only once the work has been undertaken, delivered and signed off. In addition, a 60 day invoice allows you to get paid from your client first, helping your cash flow.