USPS postage providers (like Stamps.com) and ShipStation Carrier Services providers (like Hermes and DPD) charge you for the label at the time of label creation. The minimum purchase amount for these postage providers is $10/£10.
These providers also carry a balance. So, if your balance is less than the amount required to purchase a label (or labels), you will be required to purchase at least $10/£10. Any unused funds will be added to your balance for future label purchases.