So what can credit card NX be used for?

Character Name Change, Character Transfer, and Cash Shop Surprise cannot be purchased with NX Credit, nor can NX Credit be used in the Maple Trade System (MTS).

Gachapon, Remote Gachapon, and Vicious' Hammer cannot be purchased with NX Credit until level 30.

Straight from the charge nx page.

You also cannot gift with it.
Everything else is fine with credit.