I am more likely to try things if there's free shipping (to me and free returns), but at the same time, I tend to place a premium on service and speed too. Sometimes the "free shipping" has taken ages to get to me, or in some cases - the order is cancelled by the time it gets "processed" because my size is out of stock. It was in stock when I ordered it, but then by the time someone goes to process my order, it's sold out.
I like knowing how long something will take to get to me. I'd be happy to pay a flat rate shipping fee for that kind of service. For instance, if I know it's always going to be a flat $8 or $10 or even $15 for three day shipping, I'm completely fine with that.
Having said that though, I'm kind of shocked that on some sites that don't have the free shipping option - the shipping price *increases* with the size of one's order. That doesn't make sense to me. I realize there might be more bulk, but I cannot see the overall rationale for this. So, if I spend much more, I pay a lot in shipping fees? This leads me to order fewer things and spend less on my online purchase.