thanks for the questions:
What is to prevent you or someone else using customer information in the wrong way? As you know, private information is stolen daily and is a big privacy issue.
What security can you provide?
I don't have any type of security proof I can provide to you, at the stage this web app is in it's not in my near future plans either. I could apply to the USPS to become an official provider but I don't plan to do that until I get more users. The closest thing to a security proof I can provide is that the USPS requires that every website that uses their API be registered, so they have all my personal information if you ever wanted to submit a complaint.
Also the only thing the app requires is the address, the postage is not paid so I don't require a credit card. It's really easy to steal addresses these days, I wouldn't need an app to do that.
Why would I not use the Stamps.com label utility, since I am already paying on stamps.com?
I'm actually a stamps.com customer myself. I made this web app out of curiosity and to add it to my resume really, I plan to make a mobile app as well and add many features. At the moment yes Stamps.com is better than my app, in the future I will probably also charge a monthly fee when I have more features. The plan is to make it better than stamps.com, charge less, and not lock you into a particular label supplier.