[refurb]

Your marginal costs don't go up in both cases.

If you have futures to buy diesel at $2.50/gal and the diesel price skyrockets to $4.00/gal, you can keep your prices the same.

If you didn't have futures you couldn't without taking a loss.

[ikeboy]

This is incorrect.

If you have futures, your marginal cost is still $4, since now you're using an additional gallon instead of selling it at market price at $4.

[refurb]

That's an opportunity cost, not an actual accounting cost.

[ikeboy]

Accounting wise, the fact you made a profit on futures is unrelated to your current costs buying in the physical market.

Most likely the futures in question aren't being physically settled.

Regardless, the economic cost is what determines incentives to raise price for a rational actor. For my claim above to be wrong, Walmart and co would have to be irrational.