Build, buy, or integrate
The simple rule
If the capability is why customers choose you, build it. If it's infrastructure everyone needs — payments, email delivery, authentication, search — a ready service is almost always better. The time you save goes into what's actually yours.
The assumption that “we'll build it ourselves and it'll be cheaper” ignores maintenance: security, compliance, edge cases and support continue for years after the first version worked.
What to check before buying
Exit. How you get your data out if you want to leave, and in what format. A vendor with no exit door is a long-term risk.
Pricing model. Does cost grow linearly with your success, or jump a tier exactly when you scale.
Boundaries. What the service doesn't do. Most pain comes from the gap between what it does 90 percent of the time and the last tenth that is precisely your case.
The integration itself is work
Integration isn't free: authentication, failure handling, adapting the data model, monitoring. Plan time for it, and wrap every external service in a layer of your own so a swap is a change in one place.
Going deeper
Also ask what happens when the vendor is down. An external service on the critical path needs defined behaviour on failure: a queue, a retry, a degraded mode, or a clear message. A system that crashes because a third party is slow is a system that wasn't designed, not a vendor that disappointed.