Hi PhilthyCollins,
All USPS shipping labels that can be purchased through our site will include a tracking number. If you were to purchase a second label from a single transaction, for example, the second label would include a tracking number but the tracking number would not entail an additional cost.
We have also put some security measures in place which may prevent a seller from purchasing a shipping label with destination addresses which differs from the buyer's address, so the approach to printing a label without a sale you mentioned may not work any longer.
-Mike