def days_to_mail(weekday: int) -> int:
    """
    Returns the number of days to send an item through priority
    mail within the U.S. given the weekday (Monday: 0 - Sunday: 6)

    """
    if weekday < 0 or weekday > 6:
        raise ValueError('weekday must be from 0 to 6')
    usually = 2 # days for priority mail within the U.S.
    # if approaching Sunday when no mail is delivered
    if weekday >= 4:
        # return the usual incremented one day
        return usually + 1
    else:
        # return the usual
        return usually