I wanted to point out that you can print First Class Packages with Stamps.com. Also, you get discounted postage when you purchase labels for First Class Packages using an online postage service vs. going to the Post Office.
As an example, if you are shipping a 12 ounce item:
First Class Package Service Post Office rate: $3.48
First Class Package Service online postage rate: $3.14
Savings is $0.34 per package
Plus, Delivery Confirmation is free using online postage for First Class Packages. At the Post Office, you would need to pay an additional $0.85 for Delivery Confirmation.
Hope this helps!