Cart Quantity Discount App?

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We're in an unusual position between offering free shipping and only shipping one or two products per box. (Our products are all the same size) Naturally, our cost to ship is lower per product when 2 go out at once. Or, 4, or 6, etc. We want to incentivize our customers to an even number in their cart. The discount apps I see so far are product based. I'm looking for a simple, cart based solution where we can show a potential discount to the customer who has just 1, 3, 5 etc. 

 In other words, we're going to split the savings with them. We sell more product and save a little on our shipping costs. I haven't worked out brief, clear wording but it won't matter if I can't add the functionality. Something like: "We ship plants in pairs when possible. Even up to get a(nother) discount!"