In the "Checkout Options" section of the one-page-lister the link under "Return Policy:" will read "Choose Return Policy" if you haven't yet filled in and saved that information. Once you have filled in and saved, then the link will change to "Modify Return Policy". I mention this only so that you might check this to confirm whether Auctiva has your return policy info saved in this section of the form. Of course you will also double check the info filled in, which I'm sure you've already done.
If you have done everything right here, then it should show up in your ebay listing (but not in the preview). If it doesn't then I would file a support request. Click the link at the bottom of Auctiva's Help page.