Shared IP
A sending IP used by many customers of a provider.
Most senders start on a shared IP pool. Reputation is collective: one bad sender can drag the pool down, but established providers actively manage their pools. Below 100k/month, shared IPs are usually preferable to dedicated.
reading this as early-stage startups
At an early stage the deciding factors are what the free tier actually covers, what the first paid invoice looks like, and how much of a founding engineer's week the setup costs. Feature depth that only matters at a million sends a month is noise until it is not.
Applied to shared ip, that means weighing free tier, best published rate per 1,000, entry price, and developer experience ahead of the rest, against a waitlist, a launch announcement, onboarding email, and the first transactional sends.