I am offering photo cards as a custom product. I want to be able to offer discounts on packs of 5 or 10 cards ordered, while allowing the customer to select the photos for each order. I am new to Zenfolio and still working things out.

 I think you may have to set up a custom product for each package with it's own price list.

just a crude example to keep the math simple:

Image 1 (custom product 1)= $1.00 (pricelist 1)

Image 1, 5 pack (custom product 2) = $4.50 (pricelist 2)

Image 1, 10 pack (custom product 3) = $8.00 (pricelist 3)

Then you would have to fulfill the order yourself.

If we had the option to make the coupons more flexible (quantity discounts, Buy x prints get y free, spend over x and get a y discount) things would be easier.  I've posted this in the request for new features section and so have others.