I would just list bulk quantities - you have a store and only 30 current listings, so it won't cost you any extra to have more listings (if you are subscribed to a basic store, you get 80 free listings as part of the deal - fixed price in any category or auctions in collectible categories - so you would only pay FVF if it sells and that can be factored into your pricing).

The two main reasons I suggest that is to make it nice and easy for any prospective buyers, as some won't bother and just look elsewhere, and because eBay often view "contact me for custom quantities" etc lines in listings as an ivitation to sell off eBay, whether that's what you intended or not, and issue policy violations automatically. If you do want to list with an invitation to contact for custom quantities, you need to make it so there is no doubt whatsoever that the listing will be on eBay, and don't mention fees - prices on eBay are GST inclusive and most people will want a final, fixed price quote (plus, it's against policy to pass on fees as an additional cost, so breaking it down like that could be construed as doing so), but you can issue a tax invoice with sales. 

You are allowed to have multiple listings of the same item if the quantities per listing fulfil different needs, so the quantities need to have a fairly significant difference, eg 1, 20, 50.