General question about gas octane

I was just getting gas on my way home. My car takes 91 octane gas and I always fill it up with 91 or 93 if I’m at Costco. I was at Wawa looking at how different the prices were across the different octanes offered.

87 octane - $2.85 89 octane - $3.35 91 octane - $3.49 93 octane - $3.59

From my understanding gas stations really only have 87 octane and 93 octane and if you get 89 or 91 the pump blends to two to make 89 or 91. My question is, do you need to blend them in the pump or if I filled my tank with 1 gallon of 87 octane and 2 gallons of 93 octane would it mix in my gas tank and become 91 of does it need to go through whatever mixer is in the pump?

3 gallons of 91 octane from the prices I saw is $10.47 2 gallons of 93 octane and 1 gallon of regular is $10.03

It’s roughly a 14.67 cent savings per gallon if it works. Not really sure the juice is worth the squeeze but interested if it even works

TYIA