You can allow your customers to choose a print lab in your store if you do not have one. You will take care of sending the purchased photographs to the chosen print lab, get them back and send them to the customer.
In the example, each photograph costs an extra $2.00 if the customer wants it to be printed. Install and activate WooCommerce Product Add-ons then go to Products > Global Add-ons and create a new add-on like this:
It will appear on all your photographs already. Feel free to create it only on some products instead, or exclude the global add-on on some photographs. To know more, read the Product Add-ons documentation.
The next step is optional. If you do not want to purchase WooCommerce Checkout Field Editor you can achieve the same following this tutorial. Install and activate WooCommerce Checkout Field Editor and add a new billing field in WooCommerce > Checkout Fields like in this screenshot:
This step will add a dropdown to your checkout form asking the customer to which print lab he wants his photographs to be printed. It will show always, no matter if he chooses some photographs to be printed or not.
The customer will choose the print lab also from the collections page. Just need to tick all the photographs he wants to be printed and add them to the order:
There are different ways you can achieve a similar result. For example, you can also not use Product Add-ons and use instead Checkout Add-ons. With Checkout Add-ons, you can add a checkout field with a cost, but it will be the same no matter how many photographs the customer is purchasing. This is why we used Product Add-ons, but maybe this works better for you.
Also, you can use only WooCommerce Checkout Field Editor making the checkout field required and include the print price in the photographs price. This way you do not need any product Add-on and the customer is forced to choose a print lab.