I have never bought in bulk via mail order. What is average SH costs?
Going from my old and rather feeble memory, shipping depends either on weight or cost of the item. I think the last few times I've ordered it was from around $9.00 to $15.00. Most websites will have a page to figure out shipping cost. If I can't find one, I usually just set up an account and add to my "cart", get an approximate charge, and then remove it from my cart. The few times I've had to do that, the actual shipping has turned out to be a little lower than I was quoted.
If you reload and find powder and primers, there is an aditional "hazardous" item shipping charge, but this is not applicable to ammunition. Also, I usually seem to be able to find a "Free Shipping" or "x% off coupon" that covers shipping if I order from some of the larger companies. For example, the "gunbot" website I referenced currently lists a coupon for "$12.00 off" an order at Sportsman's Guide that would probably cover most, if not all, of the shipping charges from them depending on how much you order.
Also, If you have an FFL (C&R), you may be able to set up a "dealer account" and get the ammo and shipping at a reduced charge. Graf's and Midway do this, I think J&G does too. As an aside, if you don't have a C&R license, I would strongly suggest doing this. I have more than covered the $30 cost by buying items from Midway and Graf's at dealer prices... not to mention that eligible firearms can be shipped directly to your door.