Yes, it can be costly to buy one item at a time, but a lot of the figures guessed at here are WAY off...
First to look at the example quoted above...if it cost the average Joe to ship from Fl to CA $8.70 it will cost Dwayne $7.50 , unless he has a deal I can't find.
Yes, the box from u-line is .31, and I use u-line too, but the shipping to get the box is incredible, especially for me. I just ordered $152 in shipping supplies today and the shipping to me is $57, a couple weeks ago i order $250 in supplies and the shipping charge was $77. So the .31 box is no longer $.31. Peanuts I buy in 20 cu ft. for $24 plus shipping and it's too big to go UPS or FedEx , extra charge... I get 15 to 20 packages per bag, that's about $1 to $1.25 per package and then there is the buble wrap. It's not cheap if you use the good stuff...I use 1/2" stuff so your stuff arrives in one piece no matter what happens along the way, not sure which size Dwayne uses. Tape ..yes it's just a few more cents but it adds up too.
Now to the fun stuff...you live in a rural area of Cincitucky $3.85 surchage to deliver to your home. I just sent a package from NY to PA and my cost of shipping alone was $10.02 without the box, peanuts, bubble wrap, tape, my time and the pick-up weekly fee from FedEx. Yes, if you order just one item and send it to your work address in a major city, I might come out a little ahead, but it doesn't happen as often as you might think. One more example is a member of this board, who will go un-named who is a good customer of mine...he bought 4 gallons plus numerous other items, had a fairly large bill, received free shipping, a t-shirt, a few other goodies and his shipping alone was over $50, so he wins on all accounts.
We (shippers) don't have the time to figure out each person exact cost as maybe someone shipping 2-3 packages a day could, and we have to figure on the average cost to ship a package of certain weight to almost anywhere in the USA without losing our shirts and going out of business which would not benefit anyone. The sponsors of this site try to give incentives in one way or another, but we do have our bills to pay and like to eat too. (I know I do

).
Just one more note, the USPS system does not like taking liquids and they have horrible tracking system.There is also a cost to drive there, and to wait in line, which would also then have to be added to your bill in one way or another.
OK just one more note..use a credit card that gives you rewards or cash back and just having it delivered to your door is also a discount in my book.