What are some strategies that you use to keep your shipping costs down? I keep as many of the boxes used to send items as I can, but sometimes, that is impossible. Also, sometimes boxes are far too big for the item that was sent. Where do you get cheap boxes that will work for your shipping needs?

I just suck it up and buy nice boxes from Uline. I don't think there's a good "cheap" option.

I use Uline as well, and I then pass on the cost of all material to the buyer.

Uline. You will have all the correct box sizes you will ever need to keep shipping costs down. They have perfect box sizes to snuggly fit many of the Lego set sizes. I stocked up when they had boxes on sale. Many of my boxes ended up being only $.50 each after the sale. When you're packing and shipping out 25 or more boxes a month, it's money well spent.

The only bad thing about uline is you have to order 15-20 boxes in each size you want. So if I need 10 28x18x7 and 10 18x14x5, I have to order 40 boxes for like 90 bucks. Not so great for small-time operators like me, but still better than 5 bucks a pop from ups or whatever.