Download Credits - How To Offer?

One of my sessions includes 10 digital images, but I usually put 20-25 images in their gallery.  Once they choose the 10 included images, I would love their to be an easy way for them to do a "buy all" for the remainder of the images.  

Will this be easiest for everyone if I create a 'buy all' where the images are priced at say $20/image and then give them an account credit for $200?  

Or is there a way I can credit download credits and apply them to their account?

set up a collection then add the option for additional images but i think these require a cost to be entered or , set up a collection add on and create a free download product and add that to it ?

I can't think of an easy way for them to purchase the remaining other than them just adding to cart individually.