I generally tend to ship things mid-range or better, but I can't afford to do priority on everything. So, let's say if someone wants Priority or better shipping and I can't get it for less than $3, then I would ask that the winner cover the difference. Unless it sold for 30000 credits or more, then I would do free priority.
If the winner pays through Paypal I will be more than happy to, as long as they are understanding of the fact that I am not in control of customs or the time it may take to change carriers. (Stupid that things can either move very quickly or take forever with no predictability even though we share a continent) forgive me, rambling :)