You could try putting in an extra amount for additional postage on the individual listings, and then using a flat rate postage rule to counteract it. 

On my listings, I do charge $0 extra for additional items, but I use a postage rule so that when people buy two different items, it's also $0 extra, as otherwise eBay would add the two P&H costs together. The postage rule should override the listing rule, but having that cost there to start with might disqualify the listing from being Plus... If that all makes sense