I agree! However, rather than a tab, links would be better.
When listing on eBay, it gives you a calculator. I was frustrated that this feature was not available on Auctiva, so I bookmarked the calculator. It's still a hassle to go back and forth to get the calculations for domestic, international, and different services.
I'd love it if they offered DHL! I have an account with them, and they are almost invariably less expensive for domestic ground shipping for packages over 2 lbs than either USPS or UPS. I don't ship enough to qualify with DHL for any extra discounts. They're just less expensive.
Additionally, packages are automatically insured for up to 100.00. That alone can be the deciding factor when picking a shipper, and I do this on a case by case basis to make sure I get the best price.