When you purchase a shipping label through eBay you have a choice to pay for the label with your pending funds or with PayPal.
After you purchase the label, print it and attach it to your package. Drop the package off at USPS or schedule a pickup during your mail carrier's regular delivery.
The label is prepaid (you pay for when you buy it through eBay) so you will not see charges after the fact unless you enter the wrong weight and dimensions and USPS automated system applies a cost adjustment a few weeks later.
