A participant takes the deck out of the box and shuffles it. Then they cut the deck and the top card is immediately turned over. It is, for example, the 7 of Spades. The deck is turned face-up and clearly spread to show that there are no duplicates.
The participant opens the envelope and removes the card inside. It is a perfect match - the 7 of Spades.
AND THERE ARE NO OTHER OUTS - the card in the envelope is ALWAYS the card they choose!
NO sleight-of-hand required. This literally is as easy as it gets!
Alternative endings: You can have a photo of the card posted on your Twitter feed, Instagram or other social media. Or you can WhatsApp or message them at the start and they check the message at the end.