Free photos ? but some galleries are not?

Hi

Would anyone know how to configure a free photo gallery... I still would like people to order and pay but lets say there was a situation for of a client where they can choose the images they want as they have paid for them in advance and there is no charge... ie one album you can order but not have to pay but I would not want to have 0 in other galleries? As an example a real estate needs to pick the images they need - I can just email them the ones that they chose and it is on flat rate... ideally the zip download would be alright I guess but I can seem to get it working,, also if you want to make prints that is no good... another example would a couple can choose 50 out 100 images - there is not charge.. as they have paid up front,
basically an ordering system but not a payment in this case??

am I getting to complicated

Cheers
Russell