HP printer PSC 1315 - Can't choose custom paper sizes

I would like to print my wedding invitations on my printer.

The cards are not a standard paper size but I can't manually change the size.

I can select the custom sizes in the control panel, but the button that opens a form for you to input the actual size simply isn't there.

This page from HP Product support shows exactly what I would need to do to set the page size, however in my "paper options" (located on figure 1) the Custom button isn't there.

I've tried talking to HP Customer support both on the phone and through their web chat interface, but the web chat people said that my All-In-One software is broken and guided me through uninstalling and reinstalling it.

The Phone support was completely useless and claimed that it's not possible to do on this series of printer and that I'd need to buy a new one.

Does anyone have any ideas here as how I can print this and what I should do about my printer?

I'd appreciate any suggestions you may have.

http://h10025.www1.hp.com/ewfrf/wc/...cname=c00059542&product=374568&dlc=en&lang=en