Nearly three years ago, Amazon announced that it would buy 100,000 custom-made delivery vehicles from Rivian, a fledgling maker of electric vehicles. On Thursday, after some delays, the companies said hundreds of the vans were finally on the road.The battery-powered trucks are crucial to Rivian’s business plan and survival in a very competitive auto industry. And they are an important element of Amazon’s plan to limit carbon emissions as it builds its own fleet and relies less on contractors like UPS to deliver billions of orders of toothpaste, hair dryers, dog toys and sundry other products.But questions remain about how fast Rivian, about 18 percent of which is owned by Amazon, can fulfill the retail gia …
