Here's how you can save up to 25%: add all the items to your cart, choose 'pay this seller only', pay in one payment. (although I have contacted ebay's IT department for this issue, if the discount doesn't automatically occur for items in separate listings, I will refund the difference between the amount you paid and the promised discounted amount)
With 2 items purchased, the discount is 15%
With 3 items purchased, the discount is 20%
With 4 or more items purchased, the discount is 25%