This is my best guess, and it's an easy mistake to make! When you sell a Buyer Pays postage item with Simple Delivery you press the tile for 'Get Prepaid Postage Label'. When you sell the same item with Custom Postage if you press the tile for 'Get Postage Label' then you are choosing to buy postage from eBay instead of going to the Post Office to buy a 2nd Class stamp. eBay postage is always tracked and costs more.