The Switch Harness Set is bundled, and that means fewer extra parts to source on your own.
5Pin Automotive Car Relay Switch SPDT Harness Socket Waterproof 40A DC 12V 12AWG
You will find that OLS relays give consistent results across several DIY builds over time.
You can tell the Miniduino team cares, and what stands out about their approach is the practical labeling on the relays.