Maybe make it so that adding the package to your cart actually just adds all the items. Customization then takes place by deciding which of those items to remove from the cart. It might work as a good "upsell" as people realize they need more things than they first thought.

Edit: Case in point. I was looking to get a Sparkfun Kit[1] but they were out of one item that made the whole "kit" out of stock. Down at the bottom they have a link to a "wish list"[2] that lists all the individual items in the kit. And a big button at the bottom of that page that says "add it all."

[1]: https://www.sparkfun.com/products/12060

[2]: https://www.sparkfun.com/wish_lists/96686