Gas buddy is good and I use it a lot. I don't have a Flying J credit card. I just insert my Good Sam card after the credit card I am using and get 5 cents off per gallon. I have a Pilot Flying J myrewards card, but it is not a credit card. I use it when I make purchased inside. One of these days, I may have enough points for a shower.  

As for emails, one can go to the subscriptions link that is on the bottom of their emails and adjust which emails are desired and unsubscribe from those that are not relevant.

The US National Parks card is the best thing you can have for camping in the NPs.  I'm so old mine was $10, but even at the current price of $80 it's a bargain.  Maybe not so much if you don't camp in the parks.  I gladly give up full hookups for the general peace and beauty of the NPs though, over almost all commercial campgrounds.  Some of them have full hookups.

I have a Flying J card but I don't find it all that helpful. I rarely use it unless I'm running on fumes and won't make it to the next gas sttation.  I can often find prices just as good, and it annoys me that they start dunning me to pay their bill almost before I can put their card back in my wallet.  Plus, the big scam they pulled on truckers a couple of years ago.  Generally, I try not to reward bad companies with my custom. I'm not very rigid about that all the time though.

As for Good Sams, I'm a bottom-tier member, but they send me so much email, and I mean a whole lot of email1, I never read any of them, so I don't know what the membership is good for except discounts at Camping World.

TT

We have made multiple long-distance trips. For us, the fuel discount can be significant. We are planning a trip of over 10,000 miles this year, much in the US and part in Canada. I don't know if there are any Pilot/Flying J stations in Canada, but in the US portion of our travel, it will save us money. In addition, we have Costco and Sam's Club memberships so we can pull in to one of those to get gas, usually at a better price than that of other gas stations.

Edit: I just searched. Flying J has stations in Alberta and Saskatchewan but not British Columbia or Yukon. I did not see any in Alaska either.

I have the disabled veteran version of the National Parks Access Pass.. handy thing.. also gets me free launching and parking at Fed boat ramps.. like my Favorite CoE ramp 20 minutes from the house..

We also have the Good Sam membership.. mostly because they had a lifetime membership for veterans at $50 one year at the Super Show. We have a flying J between us, and just about anywhere we go, when leaving town. Paid for itself long ago just in gas discounts. I find most parks that give a GS discount ALSO give other discounts, including military, AAA, etc.. So you may already qualify for some discounts anyways..

At $80 a year for anyone, still best deal in town if you use the Federal Lands/Parks. Remember it's not only National Parks... It's 'all' federal owned areas! 

Once a Geezer (as Lost so stated, LOL), it's Lifetime. A pass is good for up to 4 people (total count), riding in the same vehicle. 

TheGood Sam discount for campgrounds does not generally apply to us, but can be worthwhile IF you stay in campgrounds with their affiliation.

The gas discounts are good IF you travel on interstates

Vann

Now you can get them for $20 per year or $80 for life, depending on how long you think you'll be alive.  The passes for disabled and for military folks are free, though there could be a processing fee of $10.  For everyone else, the pass costs $80 per year.  To visit Yosemite it costs about $35 to get into the park, practically half the cost of the pass.

I got one as well. Never used it. Don't stay in those spots.

I've run through the gambit of all these folks selling discounts if I join. I haven't found one that matches up, except my Seniors Pass to the Nationals. Discounts for camping at any Federal owned area and typically free entry (day pass). That one I do recommend!