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I guess it comes down to folks feel safer dealing with a known payment provider and entering the details on the payment providers site, rather than on a small unkown site. I'd certainly never enter my CC details directly on someone's site, unless they were a mainstream enterprise. At least mainstream operations have security protocols in place, including who has access to those details and how it is stored. A lot of folks are also paranoid about identity theft nowadays and with some sites even requiring DOB's to process non age related orders, it's hardly any surprise potential customers vacate the cart..
Even though my current carts have shipping estimators built in, I would say that the majority of ecommerce offerings don't offer that facility. Well certainly not 'out of the box' anyway.