$20 is the standard if your getting them from Hong Kong or Japan. In fact they tend to charge $20 no matter what the size is. First find a good box. If there is realistic expense involved with finding the box. Charge them for it. Take it to shipper and get a quote. Combine that quote with the Box and give it to the buyer. It may cost $20 or less. If the seller is in the same country as I am, I would expect to pay less in shipping than I would pay for across the globe. If you are matching or charging more than international sellers that better be one nice box.
Here's a tip. Nearly every place of business has an area to receive packages. Go to that room at your work. There is likely to be several boxes ready for the trash/recycling. Ask if they would mind if you take a box.