I do use Yahoo and do as you describe. I'm in KS, so I usually pick a destination in CA and one in NY and 'ballpark' it! Seems to work ok for me - and...I've shipped to Hawaii and Alaska, and not had any issues.
If my item is below 13 OZ with the packaging, I assume 1st class. If it's above that, I assume priority mail. I use shipping confirmation for everything - including media mail.
Once in a while, I'm a tad bit short - but I'm good with that - better than having to learn a shipping calculator and/or another process.
I put my shipping into my description - no matter which site - and it works. For international folks, I require them to get a shipping quote prior to bidding. I've had 2 recent sales on Yahoo where they've not done 'this', and I've just cancelled the sale vs. trying to chase it down. International is the one that baffles me - I'm willing to ship internationally, but I do want them to 'know' that cost prior to bidding/buying.
I hope this helps!!
