If you create a shipment using our WebUI, the cost will be charged to your account when you click "Shipping" -> "New shipment" -> "Book shipment". If you create shipping labels via the API, these will be charged to your account when you use the parameter create_shipping_label=true.
The billing for shipping labels takes place after creation / parameter generation, yet usually at the end of the billing period (1 month).
Invoice date is the first day of a calendar month or the calendar day of the interim billing.
You can also prepare a shipping label (without purchasing it) in both the WebUI and API by clicking "Shipping" -> "New shipment" -> "Save shipment". The charges for these labels will only be billed to your account when you finally purchase them.
If you make payments for shipments directly to your chosen carriers, you will continue to be billed directly by them for any labels you create.