Why does ebay offer "standard envelope for trading / post cards, stamps etc." but it can't be used?

[lsjapanstuff]

I've started selling vintage Japanese postcards. Ebay offers a shipping option of about 50 cents for shipping them. Except it can't be used. 

"Ebay Standard Envelope for trading cards, stamps, postcards & coins up to $20"

You can check this option for shipping but the customers told me they can't buy the item.  So the customer (Or me) end up paying $3.00+ to send a postcard.

Or am I doing something wrong?

[disneyshopper]

The only postcards you have listed are in the Adult category, so that is why I suspect you cannot use the ebay standard envelope, as the postcards have to be listed in a postcard category.