In UK these days most ppl give cash anyway. However, I have come across the "No boxed gifts, please" one liner, right at the bottom of the invite, in the smallest font possible. Some try to sweeten it with:
No boxed gifts please, your dua's are enough...
However that's not true, cos if dua's were enough, they would have written No GIFTS please, rather than No BOXED gifts.
Personally I prefer cash, but i wouldn't write it on the card cos i wouldnt b brave enough. Part of the fun is seeing what people get u. Sometimes they r really nice gifts u normally wouldnt get for yourself.
If u dont like it, sell it on e-bay!