PostalPoint can print your store logo in a small area in the bottom-left corner of every USPS shipping label you sell, which gives you free advertising on every package. Set it in Settings → Branding → Label.
For best results, use a pure black-and-white image with no gray. Make it 300 × 150 pixels for a 300 DPI printer, or 200 × 100 pixels for a 200/203 DPI printer. Other images are accepted, but they'll be forced to black and white and resized, which might look bad.
After saving, go to Settings → Printers → Shipping Label Printer and click Print test label to check how it looks.