This isn't a suggesting on how to set this up and I am not trying to change your store model, but over the years I've done a lot of research on cart abandonment. Too high of shipping costs is the second highest reason for cart abandonment.
Here's a link to some statistics:
http://blog.rejoiner.com/2012/05/free-shipping-online-retail-sales/When I first saw your rate schedule it struck my as being in reverse to the trends - Usually the higher the dollar purchase the lower the cost. Now how this works out for me, is we offer a flat rate that takes care of the base rate of the shipping (the first pound cost). Then we add the additional poundage fees into the base cost of the items. We sell lots of items that only weigh 1 pound, so it's pretty easy for us to calculate.
Here's what I mean - Flat rate is 5.00, (that's equivalent to the first pound) Then we know that each additional pound costs another .68 on average, so we add .75 to our product pricing. On average it works out.
People purchase online for a variety of reasons, but if they see a large shipping fee they are more likely to abandon the order. On the other side, if the total of their purchase is within what they deem okay, they will continue. It is a presentation problem to get a customer to purchase and still make your profit.
So if the item is 100.00 and the shipping is 20.00 they have a greater potential of not purchasing than if you price it at 115.00 and offer $5.00 shipping. You still receive the same dollars, and the customer thinks they have a "deal" on the freight.
Anyway - just some food for thought here.
