A revenue-critical service must reach 99.99% availability ("four nines"). Today it runs two instances in two AZs with a single database — good enough for three-and-a-half nines, but not four.
Targets: 99.99% availability, critical durability, p99 ≤ 160 ms, under $1,600/mo.
Four nines means removing every single-AZ dependency: run the app across three AZs (with enough instances that losing one AZ still serves peak), and make the database Multi-AZ and add a read replica so the data tier also survives an AZ failure. A cache keeps the database comfortably in range.