If powering a RaspberryPi is as simple as plug in a PSU, switch it off may become a real problem because system needs to be halted correctly before (mainly in headless mode).

Solutions exist but may be expensive or complex to make if you're not a specialist.This solution is cheap and simple : it only uses 6 common components and 2 bargain boards.

Making would be as accessible as possible.

When RaspberryPi detects PSU off, it sends a delayed stop order to UPS and then switches off itself. UPS will then switch off itself.

If UPS detects PSU off, it will switch off itself after a specific delay to protect Lipo against under voltage.

RaspberryPi and UPS communicate through I2C bus.

See links below.