I'm at my wits end trying to figure out how to succeed on ebay.
Like others have said, stop even looking at your numbers! As jakeeangel mentions, make sure you do what it takes to avoid the big problems (obviously you already do) and don't worry about the day to day fluctuations of your 30 day numbers.
Other than the paltry discount that you can get for having really good numbers there is nothing in it for you. With average (or a bit less) ratings you will have "normal" exposure in Best Match, no restriction on your account and to buyers your 12 month numbers will look great.
Concentrate on having good quality merchandise, good looking and accurate listings, set your prices so you make a reasonable profit & make sure you are not losing money on shipping.
As for combined shipping, I think the easiest thing to do is to not try to be so specific (buy x get x% off shipping). I think it's fine to just say something like this, "we are happy to combine items to save you money on shipping, in most cases shipping for additional items will be about half the normal rate". If you just leave it like this then buyers will either just buy based on your statement or perhaps they will ask for a specific quote. I think for most buyers just knowing that you will combine and discount shipping is enough.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
- answering Trolls is voluntary, my policy is not to participate.