I test Sheetz ethanol fuels regularly as well as the regular gas - E10 and E85 are typically right on the money so no reason to think that the 88 stuff is 15%.As you might have read above e15 is rarely more than 12% ethanol
Just as e10 is rarely more than 7% ethanol
Folks really get worked up over 5% more e than normal fuel. The concerns with E in a modern car aren't to do with the seals etc. like you hear, that's v. old vehicles at this point...it's in the fuel delivery system being able to delivery the volume necessary b/c you use a lot more ethanol vs. regular gas to get the same AFR. Vehicles rated as "flex fuel" don't have anything magic beyond a fuelling system (pumps, and a flex sensor to tell the ECU how to deal with it) to run it. A modern vehicle running 5% more E isn't going to get anywhere close to taxing the fuel system w/r to providing enough volume.