Send overdue invoice reminders from a sheet

from google.oauth2.credentials import Credentials
from googleapiclient.discovery import build
import base64
from datetime import date
from email.mime.text import MIMEText
from clickruntime import on_trigger, credentials

SHEET_ID = "your-spreadsheet-id"
TONE = {1: "Just a friendly reminder that", 2: "Following up again:", 3: "Final reminder:"}

@on_trigger
def run(event):
    creds = Credentials(credentials.connection("google", "work"))
    sheets = build("sheets", "v4", credentials=creds, cache_discovery=False).spreadsheets()
    gmail = build("gmail", "v1", credentials=creds, cache_discovery=False)
    rows = sheets.values().get(spreadsheetId=SHEET_ID, range="Invoices!A2:G").execute().get("values", [])
    sent = 0
    for i, r in enumerate(rows, start=2):
        num, client, email, amount, due, status = r[:6]
        stage_done = int(r[6]) if len(r) > 6 and r[6] else 0
        late = (date.today() - date.fromisoformat(due)).days
        stage = 3 if late >= 30 else 2 if late >= 14 else 1 if late > 0 else 0
        if status == "Paid" or stage <= stage_done:
            continue
        msg = MIMEText(f"Hi {client},\n\n{TONE[stage]} invoice {num} for {amount} was due on {due}. Could you let me know when it will be paid?\n\nThanks")
        msg["to"], msg["subject"] = email, f"Invoice {num} is {late} days overdue"
        gmail.users().messages().send(userId="me", body={"raw": base64.urlsafe_b64encode(msg.as_bytes()).decode()}).execute()
        sheets.values().update(spreadsheetId=SHEET_ID, range=f"Invoices!G{i}:H{i}", valueInputOption="RAW",
                               body={"values": [[stage, date.today().isoformat()]]}).execute()
        sent += 1
    return {"reminders": sent}