Yep, in Paypal you can just go to the "shipnow" screen. Here is a shorter version:
https://www.paypal.com/shipnowOr you can click on "MultiOrder shipping" on your Paypal screen (bottom left side under Selling Tools) and that will let you create labels as well for US shipping. Via the shipnow link you can also print International Priority labels.
On the USPS website you can also create labels and pay via Paypal but you can not ship First Class in the USA. You can however ship First Class overseas via USPS
I know they are making changes to the USPS website this week though so maybe they will be adding domestic First Class?