Ammunition is not cheap to ship since it has to be send with ORM-D labeling, essentially marking it as regulated material that can be hazardous, but generally isn't due to how it is packaged or the quantity involved. It can be more expensive to ship than some things of similar size and weight (but then, keep in mind the lead involved means ammunition is heavy for its size, and that reflects in the price to ship too). That said, that's a pretty high price for just one box of ammo.
While I order ammunition to be shipped, I rarely order just one box.
Checking at Brownells indicates their price on .380ACP Golden Saber is about $25/box, so I really wonder if the place you contacted has an insanely low retail price but they're making it up on shipping (a practice not unknown on eBay- sell at cost, but charge over double for shipping and make your money there). If the shipping price doesn't scale directly with how much you buy you might still get a good deal on it if you buy a bunch (say, 10 boxes), but you'd have to check that.
Generally, premium quality defensive ammo is going to go in the neighborhood of $1/round no matter whose it is; Winchester, Remington, Federal, etc., and caliber doesn't make much difference either. A box of 20 rounds may vary only $2-3 from .380 all the way up through .45ACP.