Rivian Automotive Inc. is walking back a hefty price increase for its electric trucks and SUVs that was put into effect earlier this week, saying it will now honor prices for customers who previously ordered a vehicle. In a letter made public Thursday, Rivian Chief Executive RJ Scaringe apologized to customers for applying the price increase retroactively. He said the startup is trying to cope with rising costs of parts and materials, but it erred by applying price increases to existing orders rather than honoring the original terms.