To give a reasonable answer to your question, a few more details will help - i.e. size/weight of parcel and value (selling price) of item.
As a seller you are not permitted to charge more for shipping than the amount invoiced, so you must ship for the $20 invoiced to the buyer and you're going to have to absorb the difference yourself. However, depending on the details, you may be able to minimize your losses.
For example, if the weight and size of the item permits you to ship to Australia via Small Packet International, but the item value is fairly high (or higher than you're willing to risk in the event the item were lost or damaged), you can look around for private (third party) insurance. I use Auctiva when I need to ensure, but then I have a store and an Auctiva account. There are other insurers, and I'm sure other sellers can make suggestions.
Assuming the item can be sent by Small Packet, insurance may only be a few dollars, and you'll save at least something over the cost of $90 shipping (which sounds like Xpresspost to me).
Incidentally, if you don't mind some general advice about shipping, once you get this resolved, you may want to review the shipping options in your listings if you're selling worldwide. Some sellers love calculated shipping, others (like me) prefer to state a flat rate. You can also exclude certain problematic shipping destinations in the world that can mean trouble for you right from the start (use your "shipping preferences" options). You can change or remove those exclusions later, once you've gained some experience.
Personally I often use the flat rate to Australia as my listed shipping cost for all "International", and eat the difference on actual shipping when necessary. It usually comes out in the wash overall.
And you must tailor the shipping cost to the item cost, so for example, a $90 Xpresspost shipping cost for a $500 item to Australia probably isn't too much, but is far too much for a $100 item, and will only result in customer dissatisfaction (and negative feedback/DSRs).
You'll need to be very careful to be precise about shipping in your listings, yet keep shipping costs reasonable, until you build up some feedback. Otherwise you may find you'll be continually losing money or be blocked from selling. Start off slowly and carefully work up your profile.
If it's any consolation, even experienced sellers still have issues with shipping. I'd say it's the most difficult part of selling on eBay.
