I can't answer anything about calculated shipping (it's always been a problem for me) but maybe I can help a little bit...YES you're doing it right - highest shipping cost, and then add on the extra cost for add'l items. I sell clothing too, and I charge 4.95 to ship first class. However, I frequently have buyers with multiple wins and my add'l item price is also a dollar. So when I weigh the items, if it comes up over 13 oz I just automatically upgrade to Priority. I haven't lost anything and the buyer is happy they got their package so quickly. I usually end up with about a dollar handling fee for myself which works out fine.
I found out long ago it's better (at least in my case) not to offer too many shipping options...shipping parcel post can be excruciatingly slow and alot of buyers are unaware of that, they're just looking at the least expensive shipping prices. (and most of the time there's only a dime or so difference) So for now, I just ship first class OR priority,but I do not offer the options. My auctions are all listed as First Class (as long as weight allows)
It does take a little time to get the hang of shipping prices etc. but you will
