Just rang Click and Send to see if random policy on this has changed - and two AP reps indicated that extra cover is not available for purchase with flat rate ebay satchels. 

AP rep is seeking advice from click and send "expert" and indicated someone from AP will call me back today, as I have requested more definitive AP advice that appropriately references information readily available to the consumer of their product, such as terms and conditions or web based information. 

Edit: did not ask about large letters but hope so, cause I won't be using registered post for letters any longer if it does apply to large letters