shopping cart not showing internationally?

[rival-sky-games]

HI all, I am selling on ebay.com.au I sell single miniature figures for a board game, 99% of my buyers are buying 1 of several different listings. I am trying to make checkout smooth for them, when I use my test accounts for .com or .co.uk, there is no 'add to cart' option provided. It means any 'flat rate postage profiles' I am trying to offer do not show up! 

I am sure this topic has come up regularly, I have searched but everything Ive found is old. 

Does anyone have a list of things I need to go through to check?

Thanks all 🙂

Evan