A buyer has just purchased a book that i didn't list as an ebay plus item. Although it says free express post which is either an ebay mistake or I made an error when listing item (It would cost me more to send express than what i sold item for). 

https://www.ebay.com.au/itm/James-Follett-Torus-fiction-novel-paperback-book-science-fiction-suspens...

Where do I see ebay's rebate for the $4 express post charge on ebay plus items?

Also, is there a way to tell if a buyer is ebay plus? 

Items are automatically included in eBay Plus if you and the item meet eBays criteria, ie You're Above Std or higher, and Postage and Returns are: Free domestic postage, A reasonably price express postage option, 0-1 day handling, No additional postage charge for multiple items and Accept 30-day+ returns.

If you don't meet those criteria your item won't be part of the program.  eBay regard "reasonably price express postage" as being less than $15.00.  Make an item Express Post $15.10 and it won't be included in the program.  Other items with less than $15 Express will still be included (assuming they meet the other criteria.

Also this is not an promotion it is on going.

The only reason Ebay have the whole free postage fiasco (to my knowledge) is to compete with amazon prime which is a completely different system which actually benefits the sellers because amazon also store, send and do the packaging for the items (at a monthly cost). This is typical business practice to compete; the AFL do it by adding extra teams in New South Wales and Queensland to compete with the NRL but both teams do not draw any audiences. If the sellers are not happy with the system they simply will not use it; if ebay plus was effective everyone would use it. 

As for your problem with postage, my ebay plus items are only those above $50 and prices are marked up to cover for the loss in postage and I offer free express on all these items. I justify this by using tracking and getting the extra security that comes with tracking for all otherwise untracked letters. For otherwise tracked parcels (mine weigh 400 grams sent with stamps and a tracking sticker) it costs me just $3 to upgrade to express ($9 to $12). 

Hope this helps.

Also you might want to try sendle or another cheap courier service. With sendle for example, if you are a bulk seller you can send state to state for as little as $5 and nationally for $6.50 on the top level subscription. I have sent with Sendle and parcels always arrive within 3-5 days, sometimes next day and haven't had any issues and I think sendle are considered express.

Make yourself ineligible for the programme.

Extend your handling time (but still post within 1 day)

Or don't have free delivery.

Thank you. Best advice ever. It only took a few minutes to bulk edit all my items to "2 days handling". I will also put in my profile that items will always be posted within 24 hours. This idea also means that customers will often receive their orders before the time estimated by eBay, which can't be a bad thing........win / win.

Thanks, again.

Yay, I had a good idea