@grant

ehm yes the card is signed before blowing up the balloon... but i want to show them the card untill its tied:) so that there is no way for me to sneak it in... so after they tied it i push it in the deck.
@ace of kev
yeah but they then know there is something in from the start and wonder what it is... ofcourse they will never find out what it is

but they can backtrack how it is done ...
and indeed when they blow it up and there card is still faceup in full view:) they realize that can never be there card... but when the timing is correct and they only realize the load in the balloon after u made the card magicly jump inside .... they will be astonished;)
oh and the problem is

when they feel it inside they let the air out and feel... (that eliminates that its a card in there) etc;)
but no one has an idea? ... is it better to blow it up myself why they still have the card signed and holded:)?
because i recently came to the conclusion 20% of the people cant freaking tie a knot in a balloon and 5% of the girls blow air into the balloon and then get another gasp of air to put in the balloon (but they do it through there mouth) so it doesnt go anywhere ... i seriously waited 2 minutes for it to be the right size...