Buy more so that the shipping is a smaller portion of the cost. That is if they can actually ship the extra weight for not much more. Shipping weight is a problem for metals. That is one reason why finding a local supplier is often a good idea.
I usually check Online and Speedy, but unless it's a real oddball my local supplier can ALWAYS beat them on price and delivery- usually by 20% or more- never mind shipping. A recent example, 8" by half cold roll plate, local guy was 53 bucks including local tax, Speedy was 66, no tax, and 30 bucks to ship...
Buying more to average ship costs should work, but it presumes that the shipping costs are fixed across a range of weights. The way most of the suppliers work it, they make money on shipping/handling- prices are a percentage of total, or go up by the ounce. The exception to that rule is suppliers (like Fortal.biz for 7075) who advertise on ebay and ship using flat rate postal boxes with no or fixed handling charge. The big flat rate box can hold 50 lb of aluminum in decent sizes, and one can indeed average shipping costs down with those suppliers. Unless they've changed recently, Speedy, Online, Schupan, and the like won't use USPS flat rate boxes, even when a piece fits (and remember, those boxes are free at the PO). My opinion is that doing so cuts their profit margin (UPS negotiates enormous rate cuts for business users, so a UPS shipper can charge less than residential rates but still more than the actual cost. Note Amazon Prime's disclaimer. With any internet supplier, shipping and handling needs to be compared carefully.
Always check the local guys first. If you spend a few bucks there on a regular basis, you may find they have a drop pile or a scrap bin, too.