Discount For Overall Total Price When Using Multiple Price Lists

I need to use multiple price lists for school photos. 

I allow parents to purchase Wallet, 3R, 4R, 5R, 8R images for their child's single head shot photos and use my genera price list for this. However I need to limit the Group photos to 8R and 5R as they don't work when smaller. I have another option showing all of the students in the class with their names underneath them, much like a year book photos and this needs to be at least 8R in size to keep the names legible.  

This means I use 3 different prices lists for each school.

A further complication is that some parents purchase previous years photos as well, so those previous totals are also coming into play.

For most parents my pricing is sustainable, but with parents with 3 or more children whose orders become quite large I would like to give them a "feel good" discount.

Is there a method of applying a discount on a parents order based on the final total just before check out when using multiple Price lists that is not on a per price list basis, but on a final total cost of all photos in the order.?

Coupons do not work in this case as I do not know which parents will be ordering large numbers of photos until after the fact.

Much thanks

With both Coupons and Early Bird Special you can create a discount structure so the more they purchase the more of a discount they can receive. 

See attached. 

Or even set it so the discount doesn't apply unless the total is a certain amount. 

Thank you