For https://calenduck.co (built with Django) we use:

https://dashboard.heroku.com/ for the infra

https://www.sendinblue.com for emails

https://sentry.io for error reporting

https://stripe.com for payments

https://papertrailapp.com/ for logging

https://slack.com (free) for chat

https://trello.com (free) for keeping track of things

Totaling at 14$ a month, it doesn't really get any cheaper