If you set the listing up with free shipping you cannot add shipping even if buyer purchases multiples. You would have to ship all the items separately for free if they were purchased by different people. At least by combining them in one shipment you may get off a bit cheaper than if you shipped the individually. Of course, that isn't always the case. However, you can ship them individually, I suppose.
Surely you had the cost of shipping built into your price. If not, you need to reconsider your product pricing.
"It is an intelligent man that is aware of his own ignorance."
