I am getting a feeling that this may be an overseas purchase because you said 'For a Aust $30 outlay'.
You specified it was in Australian dollars, which wouldn't normally be the case for a domestic purchase.
But regardless, what you need to ask yourself is what digital ghost already outlined. Do these pads fit the description/Are they what was in the ad? Are they the size as marked in the ad, did you get the right quantity etc?
The important thing to realise is that just because you buy something and it does not fit or doesn't suit you, it does not automatically entitle you to a refund or exchange-not in real stores and not in ebay, not unless the store specifically allows it with eg a 30 day no questions asked exchange.
You also have the problem that if you are talking about underwear, there may be legal limitations on what can be returned or exchanged.
Normally, you are only entitled to a refund or exchange if an item is faulty or not fit for purpose. It is sounding though as if your purchase may be more a change of mind return.
If this is the case, you'd probably be better off to just accept the $5 credit because if the items themselves are fine and as advertised, the seller is under no obligation to send out larger pads for nothing. If you want larger ones, it sounds as if you'd be better off just taking the credit and buying a pack of the bigger ones as you'd be less out of pocket.
If on the other hand, the items are not as advertised in some way, you can open an item not as described claim with ebay. You may need to supply some photos to show any faults. A seller in Australia would then have to supply a return postage label so you wouldn't be out of pocket and with an overseas seller, ebay may not force you to return it, I don't know for sure.
