There are two reasons this may occur.
1. The product that you have added to the cart hasn't been enabled to "ship via letter" in your products area
2. If a product has been enabled as an envelope and added to the cart, but there are no shipping services available for letter mail. You will need to enable some letter mail services.
If however an envelope product is also part of other shippable items, such as a parcel, then the envelope item will be combined into the parcel shipment.
You can find more information about how we package items on our box packaging page.