I usually try to issue a single refund from one item if they've bought multiples, mostly because it makes it quicker / easier, and also because I worry a bit about PayPal flagging my account for too many refunds, so anywhere I can reduce the actual refund count, I will. If a single item value isn't high enough to have at least a couple of dollars spare after a full refund of excess postage, then I'll refund from two items.