I've created my USPS account, entered my info in the admin panel through System > Config > Shipping Methods > Usps, but when I select an order to ship I'm unable to create shipping labels. I can't find any documentation on what (I assume) is a simple task. Any idea what I'm doing wrong?
You can only create a label if the shipping method is the carrier which you want to ship, i.e it must be USPS in this instance. Then you will see the create shipping label tab.
For USPS in Magento the label it prints still needs to be paid for, so you have to take the parcel to the post office. Look at Endicia extensions if you want it to do seamlessly from Magento admin.