Retouching Chargee Issue

I have a group of packages and individual (a la carte) items that will all deal with a single picture. We would like to add a retouching charge as an option that will only would be charged once regardless of how many packages or a la carte items the user purchases.

I know you you make a charge at the collection level as well as the package level, but how do you prevent a double charge from happening? For example, a user purchases Package A and a la carte item 8x10.

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You can create image options that will only charge once per photo, but that option is currently not available in collections / packages.

http://www.picturespro.com/sytist-manual/articles/image-options/

I can't think of a way around it for collections.

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