I could only find a reference to it on the website and nothing in the actual postage guide pdf's. This is supposedly for all letters, yet it is not in the official guide.

 

Letter size and weight guidelines Click to expand

To be considered a letter, your item must:

  • weigh less than 500g 
  • contain flexible items**
  • have a rectangular shape 
  • be no larger than a B4 envelope (260mm x 360mm x 20mm) 
  • be no thicker than 20mm

Is your item bigger than this? We would classify this as a 'parcel'. View domestic parcel services.

 

 

I though it was specifically for those prepaid envelopes but I guess I was wrong. I think they have conflicting information on the website and in the postage guides.