If there is a significant difference in what the buyer paid for the shipping and what it actually cost I have refunded several buyers for the shipping sending out a package and I planned to use a large flat rate box because it's the perfect size for my item. I still use priority mail but may switch from calculated priority to a regional A box, a padded flat rate envelope or just a regular envelope. I always do what dhbookds suggested and that is once I have the shipping address I look at the various options available and may change the method of shipment without changing the class.

I have used it several times and Pirate Ship's rates are close to the same as the eBay discounted rate.

You can then upload the tracking information into the order. There is another option called priority mail cubic which is good for smaller heavy packages up to 20 pounds but you have to use another shipper like Pirate Ship to purchase and print the label since eBay does not offer it. Flat rate is always cheaper for very heavy items and I have shipped 50+ pounds of lead a couple of times in a medium and/or large flat rate box. As others mentioned there is no way to tell where the break even point is due to size, weight and zones.

Usps price for large flat rate box