eBay is scheduled to introduce a cart to the Australian site around November, but in the mean time it's simply a matter of confirming the purchase of each item separately and then paying at once - some sellers will have automatic discounts set up and they will be claculated as you go through checkout, but other sellers will need to send you a manual invoice to apply any combined postage discounts. (Obviously, this doesn't apply if the items are listed with free postage - in that instance you can just proceed directly to checkout once you have purchased all the items you're interested in).
To request an invoice from a seller, once you have purchased all items, you should be able to select it from the options in the dropdown menu next to the items listed in My eBay, or there will be a link to request total if you click on Pay Now and get to the first stage of the checkout.
If you are unsure about a seller's policy for combined postage and discounts, it's best to message them before purchase and ask.
The only exception to being able to purchase items individually and then paying for them together is when the seller requires immediate payment for each item, in which case you can't combine purchases from the same seller and have to pay for each one separately.
