Quick answer
The cost of your Reverb Shipping Label will be deducted from your earnings as long as you are set up with Reverb Payments. If you have a statement credit on your account, the shipping label cost may be applied to your bill.
If you use PayPal as your payment processor, the cost of the label will be added to your monthly billing statement.
If additional shipping labels are purchased on one order, those charges will get added to your monthly statement.
Further Reading:
How Do I Cancel or Void a Reverb Shipping Label?
Can I Edit the Tracking Information on My Order?
Shipping an Item on Reverb & Reverb Shipping Labels