If you're being charged $1 insertion fee for auctions, it means they are collectibles (which I would presume stamps to be, anyway ๐ ), as auctions in non-collectible categories with a store are $1.50.
Auctions in collectible categories can be free with a store, as they're included in with the free listing allocation you get - i.e. with a basic store, you get 200 free listing a month, so can list up to 200 (collectible) auctions, or buy it now listings (those ones can be in any category) before you'll be charged an insertion fee.
If you have a mix of BIN and auction listings, the best thing to do is make sure all your auctions come under the free allocation, because a BIN will be half the price if they've all been used (@50c, except for media categories, like books, DVDs etc).
Edit: The alternative, if you have in excess of 200 listings every month that you want to have, you could upgrade to a featured store, as the free allocation for those is 500, and there's a 1% reduction in FVF (at $50 per month, so it'll be worthwhile if you have around 230+ listings each month).
