As a seller, I cannot put shipping info on my auctions, as I use Priority Mail for everything, unless it's a large item, then I use UPS. We now have to know the zipcode to where it's being sent to know the cost of shipping.
Also, if the winning bidder is outside the US, then I cannot know the amt, until the winning bidder tells me where they live.
Yes, Priority mail gives you free shipping supplies, but not packing supplies. So there's still the bubblewrap cost, as well as the "hidden" costs such as gas to drive to post office, wear and tear on my computer, electric to run my scanner/computer, cost of digital camera and batteries, my time spent in listing, packing, etc.
What I do, is add $1 to all my shipping amounts to help defray the cost of all those hidden charges, not to mention my eBay fees, paypal fees, etc.
Buyers don't always understand all this, so I don't hesitate to tell them why the extra buck is involved, and I've never had anyone complain after it's explained to them.
Sometimes that buck isn't enough, but other items will end up evening it out, so I just eat the rest.
Also, if you do a search on "html postage" on eBay, you'll find a guy who wrote a javascript that you can add to your item description so that potential buyers can type in their zipcode and get the shipping amount instantly. It's really pretty cool and the guy sells it for $10. Not a big cost considering how many emails it saves you from answering during your auctions. He's also very helpful with setting it up. And he customizes it before he sends it to you via email attachment, for your zipcode of origin.