You can offer a certain number of digital downloads for free, and charge for additional digital downloads.
The easiest way to do this is to create a promo code or an automatic discount for the first "X" number of digitals for free. (If you set it up as a promo code, the client will simply enter the code at checkout to get that number of images for free.)
To offer some digital downloads for free:
1. Go to Commerce > Pricing > select the price sheet
2. Click "Discounts"
3. Click "Add New Discount"
4. Choose "Item Discount"
5. Type the promo code you'd like the client to use and click "Promo Code"
6. If the promo code should only be used one time before it becomes inactive, enter a "1" in the Limit Use box
7. Set an expiration date, if you desire
8. Click "Next"
9. Check the box for the Digital Media item(s) you would like to offer for free and click "Next."
10. Leave "Qty" as "1" so clients will receive the discount after adding at least 1 image to the cart.
11. Select "Each Item Free" and enter the total number of digital downloads you would like to allow for free in the "Times per Checkout" field.
12. Click "Save"
Be sure you've set a price for these digital items in the price sheet, so that if the client adds more than the amount given for free, he or she will be charged for the extra digital files.
Your clients will go to the gallery, add their desired number of digitals to the cart, and then enter the promo code at checkout.
All of the digital files that the client received for free (and any extra they purchased) will be automatically emailed to them!