I've tried to purchase a shipping label online a few times, and it is always a pain. USPS makes you sign up for an account, and fill out lengthy forms. You can buy USPS and UPS shipping labels through your paypal account, but that link is also difficult to come by.

I'd love to see a site when you can enter 2 addresses, a shipping weight, and a size, and then it shows you your options. It looks like this is what you're doing, so I'll definitely check you out once you release this.