Not sure which cellular service you have, but I have Tmobile and if you do too, every Tuesday they have 15 cents off/gal up to 20 gallons codes that last 2 days when you claim them at Shell stations. Also if you have the Shell credit card you get 30 cents off for the first 5 fillups, then 10 cents off afterwards. So combining them through the rewards app you get 15c + 30c off or 15c + 10c off per gallon.
Another thing I use is the app GetUpSide, you open the app, claim the discount at whichever station you're going to, then pump your diesel, then take picture of receipt within the app and you get discount back in the form of gift cards, PayPal, or check in the mail. The highest I've ever gotten from this app is 35c off per gallon. I often combine this with the rewards from above, but this app takes 2-5c off the discount if combined with other discounts. Also occasionally during non-busy hours, random stores in your area will mark themselves as having a happy hour and you can get high discounts off during those periods, typically these are like at odd hours of the day tho.
Other day I filled up, Used Tmobile Tuesday rewards 15c + 30c from credit card + (15c-3c)=12c off from the GetUpSide app for a combined discount of $0.57 off per gallon of diesel and because the fuel tank goes so long on these trucks the Tmobile rewards are available like every time I need to fill up. Diesel was $2.67/gal with discounts it was $2.10/gallon
*If you want to try out the GetUpSide app, use my referral link and you will get a bonus discount for signing up on your first fill-up, I think it's like 15c/gallon extra.
My code WILLIAM9494 gets us both a 15¢/gal bonus: https://upside.app.link/WILLIAM9494