Free shipping versus not - Any definitive studies or resources?

Hi all, just wondering if there are any good websites or studies that people have done out there to compare customers' preferences between free shipping and paying for shipping?

My issue is that our ecomm store is setup to have accurate shipping quotes for whatever someone orders, and many of our orders have to go Freight because they are very heavy. I've noticed that some of my competitors are doing free shipping on many things (some do it on everything). The issue I see though is that with our business, many people are buying multiple heavy items, and if I had to make every item free shipping, they would end up paying a lot more in the long run if they ordered multiple items versus just paying the actual shipping cost. Here is an example:

Without Free Shipping:

Item A: 200 lbs, $100
Item B: 300 lbs, $150
Shipping cost: $170
Total: $420

With Free Shipping:
Item A: 200 lbs, $200
Item B: 300 lbs, $250
Shipping: $0
Total: $450

When calculating "free shipping" I have to build in the shipping cost and assume worst case of someone buying just one thing and shipping it far from us. Now, if the free shipping thing is something that people like because it avoids sticker shock, then great, it makes more money for us because if people buy multiple items then our profit skyrockets. But it basically comes down to: Are people so attached to free shipping that they ignore total cost? Our prices will crush our competitors when people buy multiple items. I just don't know if they have the attention span to notice this, or if they just think they're getting a sweet deal and then when they see the shipping cost they puke and totally ignore the total cost?

Anyway, I know we could run our own tests but this is not just an e-comm store and with the size and scope of our operation this wouldn't be an easy thing to do. I'm more interested in seeing whether someone has already done a study like this and would appreciate any insights. Thanks.