Get rewarded for your Google Store purchases
Google One members on certain plans get a percentage back on eligible Google Store purchases in the form of Google Store credit.Google One members with 200GB plans: 3% back in Google Store credit
Google One members with 2TB, 10TB, 20TB or 30TB plans: 10% back in Google Store credit
Notes:This benefit is only available in the United States, Canada, United Kingdom, Germany, and Australia.
Google One trial memberships and plan family members are not eligible.
You must be an eligible Google One member within 30 days of your purchase to be awarded Store credit.
This appears to yield the best dollar value benefit for the Google Pixel 4 preorder (as it stacks with the $150 credit from https://www.ozbargain.com.au/node/489818) provided you are happy to spend it on the Google Store.
e.g. I upgraded to a 2TB Google One plan ($12.49 month to month) then ordered a 128GB Pixel 4 XL for $1429. This nets me a $142.90 Google Store credit (to be applied 30 days after purchase), and the $150 preorder credit (to be emailed 8 weeks after shipping).
In theory you could also cancel/downgrade once the credit is applied.
This is the way to go!