How do you deal with the customer that has received the free products and the person that has been referred having to pay? Won't the next person expect the same as the first?
You have Services and Products. The Services have a subjective value, it corresponds to your skill and the value you deliver. The Products are often times prints, and the prospect and/or customer know's the real value of it, therefore this is a bad leverage point.
The Service you provide however is again, subjective in terms of cost. Therefore you stand to gain the most leverage by charging more for the Services and giving away the Products (which cost you nothing) to get more customers that consume the Services.
There's also a justice mechanism at play here. Humans don't want to feel like they're being taken advantage of, and giving things away often elicits this emotion. It's a normal human emotion but you have to get rid of it as it's not part of the equation of selling things to humans.
If your price point of a normal shoot is $75 increase your price to $97 and "consume" the prints you give away.
As mentioned before, if you print 20x 4x6 prints at Nations Photo Lab you're really only spending $7.26 USD. But the perceived value form the customers perspective is much higher. That's where you gain real leverage.