Thanks for contributing to our forum. Once you have selected either Flat Rate Shipping or Calculated Shipping when creating an eBay US listing through Auctiva, you should be able to choose "USPS Parcel Select" from any of the drop down menus in the Domestic Shipping Services drop down menu.
However, we do not currently support printing USPS Parcel Select shipping labels so, in the event you need to ship an item to a buyer using that shipping service, you will need to ship the item directly from the Post Office or another vendor which offers that shipping service.
We hope to be able to offer USPS Parcel Select shipping labels through our site in the future, but I am uncertain when we may be able to add that option at present.
I hope this helps. If you need any further assistance with your account, please feel free to contact our Customer Support team by mousing over the “Help” tab and selecting the “File Support Case” option.