Python raw_input and conditions
Here is what I came up with:

#!/usr/bin python
from os import system as sys

def GetReceipt():
    prompt = "Insert receipt number (if you finished insert 'f'): "
    inserted_text = ''
    while True:
        sys('clear')
        inserted_text = raw_input(prompt).upper()
        if inserted_text == 'F': break
        yield inserted_text


def final(receipt):
    sys('clear')
    print ["You did a great job, here's your receipts:\n"+' '.join(receipt), 
        "You did not enter a single receipt..."][len(receipt) == 0]

if __name__ == '__main__':
    valid_receipts = [ f for f in GetReceipt() ]
    final(valid_receipts)