Chase overdue invoices automatically

import base64
from datetime import date
from email.message import EmailMessage
from google.oauth2.credentials import Credentials
from googleapiclient.discovery import build
from clickruntime import on_event, on_trigger, credentials, state, notify

@on_event
def handle(event):
    inv = event.json or {}
    invoices = state.get("invoices") or {}
    invoices[inv["number"]] = {k: inv.get(k) for k in ("email", "client", "amount", "due_date", "status")}
    state.set("invoices", invoices)
    return {"stored": inv["number"]}

@on_trigger
def run(event):  # every morning
    gmail = build("gmail", "v1", credentials=Credentials(credentials.connection("google", "work")), cache_discovery=False)
    invoices, lines = state.get("invoices") or {}, []
    for number, inv in invoices.items():
        late = (date.today() - date.fromisoformat(inv["due_date"])).days
        if inv["status"] == "paid" or late <= 0:
            continue
        firm = late > 30
        msg = EmailMessage()
        msg["To"], msg["Subject"] = inv["email"], f"{'Final reminder' if firm else 'Reminder'}: invoice {number}"
        msg.set_content(f"Hi {inv['client']},\n\nInvoice {number} for {inv['amount']} was due on {inv['due_date']}. "
                        + ("It is now over 30 days overdue. Please pay within 7 days." if firm else
                           "Could you let us know when we can expect payment?") + "\n\nThanks")
        gmail.users().messages().send(userId="me", body={"raw": base64.urlsafe_b64encode(msg.as_bytes()).decode()}).execute()
        lines.append(f"• {inv['client']} {number}: {inv['amount']}, {late} days late{' (firm)' if firm else ''}")
    if lines:
        notify.slack(f"Sent {len(lines)} invoice reminders\n" + "\n".join(lines), channel="#finance")
    return {"reminders": len(lines)}