USPS Mobile is the United States Post Office free app, and it can help you find the least expansive way to return merchandise through the mail. Plug in shipping information like to and from zip codes and some details about your envelope or package, then use the Calculate a Price feature and a list of mailing options pops up so you can find the cheapest or fastest one quickly. Extra services like insurance and delivery confirmation can be selected and will be added to the cost. You can use USPS Mobile to track your return shipment, along with many, many other features.
USPS Mobile app is available for Android, iPhone and Blackberry. Download it at USPS.com.
This app is from UPS, the guys in the brown delivery trucks. UPS Mobile will calculate shipping costs so you can determine if this is a cheaper way to return packages, and it shows you UPS shipping locations. There are also a few other features, including package tracking.
My UPS ID is required to use this app, with a specified payment method. If you don't have one, register at UPS.com. UPS Mobile has apps for Android, iPhone and Blackberry, and you can find them at the UPS Mobile site.