OK, the third-party cookies issue definitely helped a couple of our users ... but it's a major flaw in the system, for at least a huge percentage of our prospective customers.
We sell software to Mac users, and Safari is the dominant browser on the Mac -- very dominant. And unfortunately for us in this case, it defaults to disabling third-party cookies. That means that a very high percentage of our credit card purchase attempts will fail; probably much higher than those on other browsers.
*** EDIT: Chrome also blocks third-party cookies by default, and it seems IE 10 will do so, too -- not sure about IE 9. That means only Firefox ships with them enabled, and I would guess that will be changing soon, given the others' defaults. ***
Given that this issue is on the fatfreecartpro page, can you at least put up a big obvious warning box, explaining that users will have to allow third-party cookies, in order for the transaction to work? Ideally, this warning would contain a link that led to an explanation on how to do that in the major browsers, too.
But this is just a short-term fix suggestion; long term, using third-party cookies isn't acceptable, due to the security issues. If e-Junkie can't improve this process, we're going to have to figure out another way to accept credit cards (does this same issue exist with PayPal Pro?) ... and that would probably mean we'd need to start looking for a different cart solution, as we can't mix two systems together on our checkout page.
We've been very happy with e-Junkie thus far, and would hate to have to leave ... but we can't have a credit card system (where users are sometimes rightfully cautious to begin with) that requires a user to change a default browser setting