There are a few steps involved to ensure your product is shipped in an envelope / and quoted as letter mail.
Envelopes must be enabled as letter.
Products are set up/ enabled to ship inside a envelope instead of a box.
Letters will only be provided if the weight and dimensions of your product fall into the requirements of letter mail, the packing system has selected a envelope ( envelope enabled as letter ) to pack your product/products.
For more information please see the Letter Mail requirements.
If a parcel mail custom box ( not a custom envelope, predefined envelope ) is selected to pack your product a letter mail option will not be provided.
If a customer visits your website and selects a product that falls into a letter option (weight and dimensions are applicable to letter mail, product ticked as send in a envelope instead of a box, is packed into a envelope) the letter mail option will be displayed.
Letters are packed to the total maximum weight requirement only for letter mail using the Australia Post API. The API will not allow multiple envelopes to be added in one shipment if the shipment is larger than the maximum weight.
As you can see in the screenshot below. A rate is offered for 'one' product only and the products weight is set to 0.5 kg.
If the customer was to add 3 products into the cart (combination shipment:1 letter mail option and two parcel options) the parcel price will be offered at the cart / checkout.
You will need to select the envelopes for the packing.
The shipping services for letters will need to be enabled.
In the product area, the option to send the product in an envelope instead of a box will need to be enabled.
SEND AS LETTER: TICKED
How can l see which container\containers were used to pack my products?