Although, aside from development costs, and potentially training support staff to help with any issues or questions, there shouldn't be too many costs, especially if they were to develop their own payment processor.
There is also an additional issue for Amazon to accept bitcoin payments. Most of the time, when I buy something from Amazon, the package will come in an Amazon box from an Amazon warehouse (sometimes, the package will be shipped directly from a third-party seller). In any case, the majority of the time, Amazon is operating only as a marketplace, not unlike the marketplace on the forum (although with many more rules and regulations, and at a much larger scale). Like the marketplace section on the forum, there is a lot of fraud, fake trades, and fake feedback. Amazon has invested a lot into weeding out that fraud, part of which includes forcing everyone who buys on Amazon to provide a real credit card. Allowing people to pay via bitcoin could potentially throw a wrench in their fraud detection models, which could be very costly, even if no fraud happens via its bitcoin customers.