I tend to ship USPS Priority for everything, I find it to be the cheapest and fastest. If you go on the website, you can order an array of different size boxes, by the bundle of 25, shipped right to your door, for FREE. It is a very good deal. If you need boxes in other dimensions that they do not have, Wal-Mart has some decent selections and they are cheaper than Uline. If you get into some serious shipping, like daily, then it makes sense to order from Uline. Uline is only economical if you are ordering a freight-sized amount of stuff or you will spend almost as much on shipping the shipping supplies to yourself as you do on the supplies themselves. Enormous bags of packing peanuts that barely fit through a household door are 27 bucks each vs. the tiny ones at Staples for 45. Freight will run probably about 75-150 depending on where you live. I live in Atlanta where there is a Uline(but also a 2.5 hour each way drive for me) and have actually had the shipment arrive the same day as the order!
I ship often, multiple times a week, and I try to get at least 6 month supply of materials at a time so that I don't spend too much on freight. I have a dedicated shipping room in my house so I can store it all out of my living space. If you are not on that level, one thing to try would be to track down peanuts locally. Most "gift shop" type businesses get a lot of things shipped in and then they have to figure out how to get rid of the peanuts. Often they are happy to save them for you. Starting a relationship like this with just 1-2 places could supply you indefinitely and also be recycling. Sometimes people save them and sell them relatively cheap on craigslist. Anything more than $1 per cubic foot is a scam.