Actually, you have the ability to use USPS PayPal (Click-N-Ship) shipping from any ebay auction or sale. If the buyer paid PayPal, you can go to PayPal and select Shipping on the item or do it from the ebay Sold group in My Ebay. Here's a link to ebay's info on the subject.
http://pages.ebay.com/services/buyandsell/shippingcenter9.html
You can also create a shipping label for any package or purpose directly in PayPal, e.g. non-ebay sale from this link.
https://www.paypal.com/us/cgi-bin/webscr?cmd=_ship-now
I started out simply printing the labels to paper and cutting-taping the label to package to understand the process. I next found some very inexpensive labels at ebay from a discount seller and have never looked back since.
To setup calculated rate shipping in an auction, you need to select that method with package info, weights, method, etc. in the listing editor on Auctiva. I would recommend you study USPS shipping rates and methods and simulate what you'd want to do by using the USPS shipping calculator on the usps.com website to see what methods are accepted for a particular package/item. That always gives me a good stratification of the various options and conditions that I can then use to select my options for listing. I often allow Media, First Class (13 oz or less) and Priority, whenever possible, but it depends what you are selling. You also need to consider any packaging/handling costs as add-on, but be very careful to EXPLAIN those, as buyers are often leaving bad DSRs without good explanations.
Dan