How do I combine shipping?

[shoppingbag*]

I have items for sale with fixed price Australian postage $7.60.  Some of these items would fit more than one in the box, but would then go over the 500 grams and then postage becomes subject to the buyers location.  I don't have a problem YET as no-one has asked, but I feel it will happen soon, that someone will ask about combined postage. Firstly, should I be putting it in my listing that I will combine postage if it is the cheapest option, and do I have to change something in my Site Preferences to allow this? Or does that just apply to Sellers who are selling more than one of the same item? Ok, possible scenario, two of the items will post to the buyer"s location for say $10.40, I think I can send an invoice by choosing one of the listings ? with the new postage and that should be fine.  Next problem, I want to print a shipping label, will this print out correctly with the new postage weight etc, may have to even be a bigger box.   Then what happens with the tracking? it is now two or more listings rolled into one, what do I do with the other one or two original listings?  Do I just mark them all as posted, same tracking number?  That just doesn't sound right.

Second scenario, someone in the furtherest location from me wants to combine postage or two items but instead of two times $7.60, the postage cost will be over $16.40  Is it acceptable to just explain the situation to the buyer, that it is not feasible?

I'm sure you experienced Sellers out there know all the answers, so please help, it would be greatly appreciated.  I would like to know what to do before the situation arises.  Thanks to all.