Shipping from Marco would be more than the cost of the door. Add a ten pack of machine screws and you can double or triple the postage !

Well understood.  Marco's shipping cost, whilst I have found has improved recently, is still too expensive.  My point was, if you don't need a new door skin, then buying from either Marco or RTBB is unnecessary.  I would suggest first trying to find a used but good condition one.  Parted out machines are a common occurrence in America; finding a used door skin in Australia would probably be more challenging.

US shipping has at least doubled if not tripled in the last couple of months, there is no way you could get a coin door posted here for $20
I was quoted $65-$69 for postage only on some t-moulding for an arcade cabinet from a few different US suppliers

We're talking about shipping a coin door skin, not the whole door.  The most economical way to shipping something of this size and weight would be USPS First-Class Package International Service (https://www.usps.com/ship/first-class-package-international-service.htm).  Not trackable, but at least very reasonable.  Length + width + height must not exceed 36", and max weight cannot exceed 4lbs.  Door skin easily meets this criteria.  Cost on calculator to send say a 1.5 lb item is about $20.

I have used this service quite a few times for things that fit into it, provided the seller is OK to send it this way.  For private purchases, never had an issue.  For sellers like Marco, they will probably only go with an option with tracking, and now you're talking alot more cost.

No harm in putting the want ad out there as suggested on the American forums - there's a significant more amount of stuff there than here.  I have seen in the past used door skins going for about $20-30.