I am sorry you have lost functionality for custom print sizes and this is the first time I have heard of it being "removed from Windows 8." I have found a lot of success using alternate print drivers in regards to this situation but I have only had success with using this on Windows 7. So I want to offer a suggestion that I really hope works for you.
Start by searching for the words "Devices and Printers" using your computer's search function to access the familiar page where all of your printer's icons are stored. Then proceed by following this document carefully if you have already installed your printer onto your computer.
--Assigning Alternate Print Drivers-- I recommend the Deskjet 990c driver for this and let me know what happens. Even if this doesn't work, I will do my best to help you get this to work for you one way or another. This is simply my first suggestion.
Keep me posted on the results or any error messages. If this does not help, is the Hallmark Create a Card the only software you need this to work on?