nuccionino, your shipping rates are excellent. And I agree with you that doing free shipping doesn't work for a lot of sellers and/or their buyers.
The only way I found to combat low DSR's is to sell more items. Since most buyers are decent folks and leave high DSR's, the more you sell the more likely you'll dilute those low DSR's, and thus you'll never have a problem. In my experience if you sell 100+ items a month, you'll be okay.
In addition, it is possible to find out who left you the low DSR with exact precision. The method is described on an eBay forum. (if you are interested I can PM you the link) It seems complex at first, but once you get the hang of it it is simple and easy to check for DSR's on a daily basis. You will know exactly who left you an exact DSR number in every category.
Once you find a buyer who's leaving you low DSR's, even just 3's or 4's, you can block them. Once you have a pretty good sized blocked list you will reduce your chance of getting those BS low DSR's. It is against eBay policy to discuss those low DSR's with buyers, so make sure you never do that. I just block them and move on to better buyers and better sales. Don't focus on the negative.