Must garden until 2/1/2022 to hit my goal AAOA. 
MCC Code 5411
| Amazon Fresh | GROCERY |
| Dean & DeLuca | GROCERY |
| Giant Eagle | GROCERY |
| Kroger | GROCERY |
| Sheetz | GROCERY |
| Target | GROCERY |
| Walmart Grocery | GROCERY |
| Wine.com | GROCERY |
Edit: 10/2/2021 - Fluz now disallows the Paypal Key - their system "sees" it as a prepaid card, and will not let it be added. My PPK that was previously added was removed by their system. 
I got an email from Fluz notifying me that their rewards scheme has changed. Now, there's an added reward percentage for using a non-credit card payment source:
Get ready to earn up to 3% more, app-wide. Your cash back rates just got a major update. Use a payment method without credit card fees to earn up to 3% more than before, every time you pay with Fluz. |
Basically the best discount is for linking your bank account and paying via ACH, but there's also an additional discount for linking a debit card. For 'grins' I linked my Paypal Key Mastercard, and as I had hoped the Fluz app recognized it as debit. 
To see if it would actually work I purchased a Walmart gift card with Paypal Key, using my USAA Limitless as the funding source, and it worked - my Fluz rewards were 1.5%, whereas the rewards are only 0.5% when using a credit card. Combined with 2.5% on the Limitless, that's a total of 4% back for Walmart, not too shabby for someone with significant Walmart purchases.
I made another purchase using my BBVA Clearpoints to see how the charge will (eventually) code... Fluz is said to pass the correct MCC, but since it's also looping through Paypal I'll honestly be amazed if it makes the journey with the right info. Since Paypal Key transactions can take a few extra days to post it might be next week before I know for sure, but either way I'll come back and report.
There are some caveats with Fluz... you're buying a gift card, and of course there's always some (slight) risk when doing that and you'll lose any extended warranty, etc. that a credit card would provide. But for many situations this could still work out well (assuming Paypal doesn't nerf the key, of course).
There's also this about Fluz rewards... back in September, they changed how they payout. You now have to earn a one-time $26 before you can get to your rewards. From the site:
UPDATE: As of Sept. 23, 2020, we recently updated the $26 lifetime cashback requirement to include Fluz balance payments. You must hit the minimum requirement of lifetime earn of $26 first before you can withdraw it or use it to buy more gift cards. This is a one time requirement only. After you've hit $26 in earnings, you may withdraw anytime even if it's just $1.
https://help.fluz.app/hc/en-us/articles/360022359771-Can-I-pay-with-my-available-Fluz-Pay-balance-
I checked the app, and I'm actually over half-way there without even trying (from purchases made last summer/fall during the Amex promo) and in any case this won't be a significant hurdle to reach for those who find this beneficial.
Sorry for the length of this post, but I wanted to provide as much detail up-front as possible. 
Edited to add: Write-up from Doc - https://www.doctorofcredit.com/fluz-app-buy-exact-value-gift-cards-at-a-discount/
I got into the paypal key game a little too late. In fact, so late that I haven't even had the chance to use it.
I also kind of abandoned my Fluz app a few months ago -- sort of lost interest. I still got another $5.15 to go before I can make my first redemption. I'm going to try a Starbucks reload with PP Key and see what happens. I want my $26 
Never heard of Fluz (sounds like a disease really). Does this mean I could fund Paypal Key (3%) with my Chase Freedom Unlimited (1.5%) and then use Key to fund Fluz (the exact gift card amount) and get 1.5% back on Amazon gift card?
If so...what the hell math did I just do? How much would I be earning in that scenario? Is it 3.38% cash back?
@UncleB Not sure how you made it work, but I added my PP Key to Fluz and it's showing the same rate as a credit card. It's not getting recognized as a debit
@imaximous wrote:@UncleB Not sure how you made it work, but I added my PP Key to Fluz and it's showing the same rate as a credit card. It's not getting recognized as a debit
I added it as a debit card (not credit)... I just checked, and it's still working for me:
With my Paypal Key debit Mastercard:
This is with an Amex credit card:
@imaximous wrote:@UncleB I did select "Add Debit Card" when choosing the new payment method. Not sure why it didn't work for me. I may have to get a new PP Key and try again later. 😔
Hopefully you'll be able to get it to work. If you can't, even without the Paypal Key I can still see some interesting possibilities since Fluz is now supposed to report the appropriate MCC for many gift cards.
https://help.fluz.app/hc/en-us/community/topics/360001819452-Stacking-It-
This in particular could be interesting:
Which merchants on Fluz will code as GROCERY?
"Walmart Grocery' is curious, since as far as I know Walmart Grocery doesn't take Walmart gift cards for payment (it's been a while, so this might have changed). ¯\_(ツ)_/¯
Either way, Walmart Grocery has a slightly higher (0.5%) reward rate over 'regular' Walmart gift cards, so if they're interchangeable that could be useful even without Paypal Key assuming they code properly as 'grocery'.