I'm posting the answer on the forum, as there are lots of us selling astro-bits to each other, and it is a general issue that is worth covering.
Firstly, you can calculate postage with Australia post from their website for parcels. Link here:
http://www1.auspost.com.au/pac/
They used to have a neat little PDF with postcode zones, and pricing structure so you could work out the "most expensive" zone that you could possibly send to, and use that as your baseline figure. I can't find it anymore, and the version I have is 2 years out of date.
The maximum parcel weight with Australia Post is 20kg. Note that the hardest part is that they will either weigh it, or measure it's dimensions, and charge you the higher rate.
If you send registered post, you automatically get insurance for up to $100 of value. You can also purchase extra insurance for $1.20 per $100 of value.
Knowing all of that, if you need to put a price, calculate the furthest place from you that you're likely to send it, and put that in as a postage price, but you can offer "adjustments" for shipping on eBay if you can get the buyer to tell you what postcode you are sending to. Alternatively, invite them to ask a question on pricing to their postcode and email them.
Don't forget to factor in the packaging costs (ie box, bubble-wrap, tape etc etc). My wife sells a lot of stuff on eBay, and the HUGE rolls of bubble wrap we have are scary (she purchased a bulk lot which was 2 rolls that are 6ft long and 3 ft wide)
If you're sending anything even vaguely fragile, or expensive, then I would definitely send it registered and insured, and put fragile/glass stickers on it if it is a scope or something like that. I work behind an Aussie Post delivery centre, and they way those guys toss parcels around, I would be very sure to package it very, very well.
As for your specific item, if you have the original packing foam inserts, I would highly recommend shipping it that way.
Turbo