That is why I always keep a supply of U-line 15x10x6 boxes on hand. They will hold most anything from NIB Nikes to Allen Edmonds. And, like @Brianpore said, they still usually come in under 4 lbs. So, you will generally be in the $5.95 to $15 dollar range.
Late to the discuss, but I buy five 16 x 12 x 10 boxes from Office Depot (they're like 50% off if you buy at least 5 - otherwise their prices for boxes are prohibitive), which also have pre-marked fold lines so you can cut them down to smaller sizes really easily...and work great for boots, shoes with original boxes, or other odd larger items that won't fit in USPS boxes. Probably cheaper to buy bulk from ULine but I'm lucky if I actually use 5 of them so I pop into the store when I run low once or twice a year.