I offer combined shipping, which is fine, for satchels. However, many of my items are sent via large letter. Some can be combined, but many can not, only one will fit in the large letter, and two large letters is still cheaper than a parcel. 

Nonetheless I constantly get people asking for a combined invoice for two items, both of which are listed with $2.50 postage costs. I don't know what to say to these people. The truth is I CANT combine them, but I feel like, if I say that, I risk coping a low DSR for postage cost or even a neg for not combining because buyers don't care how australia post handles letters and packages, so at the moment I end up dropping a few dollars out of my profit to discount it for them.