Send Xero payment reminders that get firmer over time

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_trigger, credentials, http, state, notify
XERO = "https://api.xero.com/api.xro/2.0"
TENANT = "your-xero-tenant-id"

def xero(method, path, **kw):
    r = http.request(method, XERO + path, **kw, headers={"Accept": "application/json", "Xero-tenant-id": TENANT, "Authorization": "Bearer " + credentials.connection("xero", "company")})
    return r.raise_for_status() or r.json()

STAGES = {7: "a friendly reminder", 14: "a second reminder", 30: "a final reminder before we follow up by phone"}

@on_trigger
def run(event):
    invoices = xero("GET", "/Invoices", params={"Statuses": "AUTHORISED", "where": 'Type=="ACCREC"'})["Invoices"]
    sent_stages, gmail, sent = state.get("stages") or {}, build("gmail", "v1", credentials=Credentials(credentials.connection("google", "work")), cache_discovery=False), []
    for inv in invoices:
        days = (date.today() - date.fromisoformat(inv["DueDateString"][:10])).days
        stage = max([s for s in STAGES if days >= s], default=None)
        email = inv["Contact"].get("EmailAddress")
        if not stage or not email or sent_stages.get(inv["InvoiceID"], 0) >= stage:
            continue
        msg = EmailMessage()
        msg["To"], msg["Subject"] = email, f"Invoice {inv['InvoiceNumber']} is {days} days overdue"
        msg.set_content(f"Hello {inv['Contact']['Name']},\n\nThis is {STAGES[stage]}: invoice {inv['InvoiceNumber']} for "
                        f"{inv['CurrencyCode']} {inv['AmountDue']:,.2f} was due on {inv['DueDateString'][:10]}.\n\nThank you")
        gmail.users().messages().send(userId="me", body={"raw": base64.urlsafe_b64encode(msg.as_bytes()).decode()}).execute()
        sent_stages[inv["InvoiceID"]] = stage
        sent.append(f"{inv['Contact']['Name']} {inv['InvoiceNumber']}: {inv['AmountDue']:,.2f} ({days} days, {stage}-day reminder)")
    if sent:
        notify.email_me(f"{len(sent)} payment reminders sent", "\n".join(sent))
    state.set("stages", sent_stages)
    return {"sent": len(sent)}