You are the only one who can answer that question.

If you sell multiples to one buyer, do you want to give them free post, or just a nominal amount?

If you have enough "fat" in the sale price on multiples can you cover free post? Or do you charge one fee and bump up enough to cover the bulk and weight of the quantity being sent.

Buyers don't want to pay post cost for each item, so you need to know how much for 2 or 4 or 6 etc.....weight and perhaps keep it reasonable so that they feel they have got a great deal.

You can probably set up a special for multiples for your listings too. That would make it easier for you and them as they would see straight away what their cost is for post based on quantities purchased.  

You should know what to charge from your previous sales.  

Why would you think eBay would pay the postage for you? You are responsible for your business - no one else.  

Good Luck going forward.