I also buy boxes from Uline, but only for unusual sizes (think 24 x24 x 6 for vintage bread boards). Shipping boxes is rather expensive.
I use flat rate USPS boxes generally when heavy items travel a far distance, otherwise I often use USPS Priority Mail in various sizes (order online for free if your post office doesn't carry them all). Also order free USPS Priority Mail long stickers to cut down on your packing tape use.
Finally, I recycle boxes if they are in good condition (Amazon and other sturdy boxes). With over 950 sales I've never had a customer complain about my use of recycled boxes. In a pinch, I've bought boxes at Staples.
Good luck!