Adding letters to the end:
* Cardboard
* Cardsharp (someone skilled at cheating at cards)
* Cardamom (a spice)
* Carding (the process of combing wool)
Adding letters to the beginning:
* Scorecard
* Playing card
* Greeting card
* Credit card
* Postcard
Other possibilities:
* Wildcard
* Index card
* Business card
* Calling card
Let me know if you want to explore any of these further, or have another word in mind!