from datetime import datetime, timedelta
from typing import List, Dict

def group_envelopes_by_date(envelopes: List[Dict]) -> List[Dict]:
    """Group envelopes by date and add headers for each group."""
    grouped_envelopes = []
    today = datetime.now()
    yesterday = today - timedelta(days=1)
    start_of_week = today - timedelta(days=today.weekday())
    start_of_last_week = start_of_week - timedelta(weeks=1)
    start_of_month = today.replace(day=1)
    start_of_last_month = (start_of_month - timedelta(days=1)).replace(day=1)

    def get_group_label(date: datetime) -> str:
        if date.date() == today.date():
            return "Today"
        elif date.date() == yesterday.date():
            return "Yesterday"
        elif date >= start_of_week:
            return "This Week"
        elif date >= start_of_last_week:
            return "Last Week"
        elif date >= start_of_month:
            return "This Month"
        elif date >= start_of_last_month:
            return "Last Month"
        else:
            return "Older"

    current_group = None
    for envelope in envelopes:
        envelope_date = datetime.strptime(envelope['date'].split('+')[0], "%Y-%m-%d %H:%M")
        group_label = get_group_label(envelope_date)
        if group_label != current_group:
            grouped_envelopes.append({"type": "header", "label": group_label})
            current_group = group_label
        grouped_envelopes.append(envelope)

    return grouped_envelopes