If you have InDesign, then export the cards as a high-res TIF, an AI or EPS file, and reference the unique file names for the card in the merge file. Same with PagePlus if you have it, but use an image type instead of vector (though AI should work, I haven't used it in PP). Set up your page size for the card size. If you do not need to merge the card design, you can use Word or LibreOffice (LO is free).

Mike