Effect: 

The performer introduces a small envelope and shows that there is a playing card inside. He places it on the table and never touches it again. 

A participant takes the deck out of the case and shuffles it. They then cut the deck and the top card is immediately turned over. It is, for example, the Seven of Spades. The deck is turned face-up and cleanly spread (on the table or in the hands) to show that there are no duplicates. 

The participant opens the envelope and removes the card inside. It is a perfect match. There are NO other outs - the card in the envelope is always the card they choose. 

Alternatively you can have a photo of the card posted on your Twitter feed or other social media, or you can message them at the beginning and they check the message at the end. 

Works 100%, Fools 100%. Zero sleight-of-hand and zero memory work.