If you are charged by the label-platform when the label is created or only once it is scanned in the post-office depends on the carrier itself:
The majority of carriers are prepaid carriers (e.g. USPS)
charge occurs when the label is created
and postpaid carriers (e.g. UPS and Fedex)
charge occurs only when the label is scanned
Please check with your carrier and label-platform directly to confirm if your carrier is pre- or post-paid.