Three examples: Authentication, payment processing, invoices: https://thebootstrappedfounder.com/not-in-house-on-reinventing-the-wheel/ https://thebootstrapped

Three examples: Authentication, payment processing, invoices: https://thebootstrappedfounder.com/not-in-house-on-reinventing-the-wheel/ https://thebootstrappedfounder.com/not-in-house-on-reinventi... I would add things like sending newsletters (Mailchimp), monitoring (Sentry), various types of analytics (Segment, Hotjar) to the list.