Yes, you can use any deck...that you've prepared! ;-)
I reread the ad copy...it does say 'any old deck' which is true.
You do have to prep it first tho.
You are correct again...this is a fantastic trick.
Easy to prep/learn/perform...the reactions are great and the double signed card is a great souvenir.
fyi...giving that one card away as a token doesn't 'ruin' the remaining deck. You can still perform the trick with the remaining deck minus those give aways.
Best wishes...