PhotoDay accommodates combined checkouts, as long as the customer is ordering from the same job.
This is especially helpful when using PhotoDay's Private Galleries, as customers can add packages and products to their cart for one private access code, then visit another private access code from the same job, add more packages and products to their cart, and checkout only once, resulting in only one shipping charge.
Customers can access the first subject's gallery, pick their favorite photos, and select their products like normal.
But before they click "Proceed to checkout" click the X in the top left corner, then the menu button in the top right corner, then "Galleries." From the Galleries home page they can either select the next subject for which they'd like to shop or click "Add Gallery" to enter a new access code.
In this second gallery, they can repeat the shopping process, and when they're ready to place your order, they will see a combined cart from both galleries!
Then complete the checkout process as usual, and the photos will be on their way!
It is not currently possible to combine checkouts from entirely different jobs (like separate group/public galleries).