Don't be Stupid; Buy the Best
Simple idea: When building an early product, use 3rd party services for everything except core value/product, and use the best ones.
Said better by Johnathan Nightingale:
"The First Rule of Startup Club: if you’re not planning to be the best in the world at something other people already do well, then don’t mess around with implementing your own version of it, use theirs" - src
The math makes sense: A 3rd party has economies of scale you can never have, so unless there's a market failure, there's you can't build the same product/service in-house for less.
~Lyon