This sounds like an incredible feat. If the envelope contains the same amount of change that matches a number he creates, it would be amazing.
Have the person open up the calculator on his phone. Ask him to enter any number from 1 to 10 into his calculator. There is no force; he may enter any number he chooses. Then, after a little by-play, he totals his answer. You remind him, had he selected a different number, the result would be different.
You ask the spectator what his total is. He says 2 1/2. You appear to be confused and go over the math again with him to ensure no mistake was made. The answer is indeed 2 1/2! The spectator thinks you have failed.
You ask the spectator to pick up the envelope and empty the contents onto the table. Much to his surprise, there are two pennies and a half of a penny! The envelope contained 2 1/2 cents -- your prediction is correct!
This unexpected ending always gets a big laugh and great reaction. It is totally self-working and a clever bit of business. NO SKILL REQUIRED!
Actually, no calculator is necessary. The by-play with his chosen number is so simple to do, it can be done in his head.
Comes with all needed coins, including a half penny and coin envelope with full directions. Fits in your wallet, so it will always be with you. Easy to do.