"We spoke to the Tech Team about this issue today. This happens when the size in the item specifics is larger then the recommended size for the letters.

For example if you are selling a coat and the length is 60 inches. The system sees this as too big for a letter and don't show it as an option."

So if I am listing a necklace 18" long I'm never going to be offered the Letter option, even though it fits in an envelope just fine? Really?