It's a good idea if you do it right. Whatever you do, don't right anything sappy or wussy in the card

The last thing you want is asking her out and getting the response "...but I think of you as my big sister." At the same time you don't want to insult her. A slap across the face can be embarressing in a shop

Try and find a good in between. Something cheeky that will get her thinking......why?.......Think about it. If you hand a girl a card and she says "awwww...how sweet" she will think it is nice, put it down and get on with her day not thinking about it (only works a treat when in a relationship).
But, if you hand her a card and walk out of the store. If you look back and she is laughing (with you ,not at you), slightly confused, still reading the card or something along those lines, then you are on the right track. Show that you are not afraid to speak what you feel. If you try and say something in a way that you think she wants to hear it, she will pick up on this in an instant and be immidiatly turned off....trust me.
If she is thinking about that card and you all afternoon, she will ask herself "Why am I thinking about this person.".....

.....instant attraction.
As Arnorld Shwarzasomething would say: "Don't be a girly-man."
http://www.wilk4.com/humor/humor.htm - Has some funny stuff, you might get some ideas from here.