I think bundling products is always a good tactic to try, of course you have to be smart when choosing what you bundle, because the products need to accompany each other well to be effective.
Combining two or more hats, definitely worth trying. I don't know the hat market at all well, so the question of success would be do woman buy more than one hat. If hats are like shoes, than I assume the answer would most definitely be yes lol!
What about hat accessories, if they exist lol. Even hat storage and those types of items. Things that value add to the hat itself, these might make good products to bundle, while giving you a further point of difference from the competitors.
Big race days are always popular, what about doing something seriously different like offering a race day pack. Hat, matching clutch and even mini binoculars or a small hand held radio to hear the call with lol. OK, maybe not those exact things, but you get the picture. That's called themed selling, and can get some good attention if it's kind of quirky enough :)
One product I would definitely be taking a serious at if I was dealing in hats would be belts. While I confess to being seriously fashion challenged, I think belts would definitely coordinate well with hats.
In regards to your ebay supplier, if you're in a position to buy in volume then why not talk a better price with them. You will even save on the shipping as well, so that would build in some extra margin.