I've been ordering from McMaster-Carr for a few decades and never had a complaint about the shipping, it was always far cheaper than any other place I ordered from. Prices from UPS have gone up significantly this past year, so ordering online is not the deal it used to be. I don't recall UPS rates being based on package size in the past as long as it was below the oversize category, but I just tried a sample shipment with two different box sizes for a 10lb weight and the larger box shipping weight went up to 16lb with a corresponding price increase. I try to combine as much as I can when ordering from them now, in the past I would not hesitate to place an order for a single item since the shipping price was so cheap. If shipping costs might be an issue for an order, I will plug in the weight in the UPS rate estimator to get an idea of what the costs will be, but it would be nice for them to be able to add that to the order page so I don't have to do that.
Their steel prices are pretty good, and there is a good selection of grades. I placed an order for several sizes of 1215 in 6' lengths to keep on hand for various projects and even with the shipping charge, they were cheaper than any other place I could find, including my local steel supplier. I also like that what they sell is a quality name brand. I have yet to get a poor quality product from them, and their customer service is some of the best in the industry.