Will there be some sort of seasonal/occasions model in addition to the monthly one? Because I could see this being incredibly useful around Holidays (i.e. Christmas) and having an automatic Christmas card generated and sent out to all my family and friends.
At this point you can use the pay as you go option to send prints with flexibility. We are going to add a slick one-off printing and sharing interface over the next month. We are looking to experiment with ideas around the holiday season.