Amazon currently does not honor its prime shipping agreement with all domestic customers and has no customer service path to get the issue resolved. In some places USPS is unreliable and Amazon insists on using it repeatedly, violating its own promise to customers. This suggests to me that Amazon faces no competitive pressure to deliver good customer service.
As much as I don't want to be a fan of Amazon -- their offering, especially because of the logistics/shipping, combined with generally fair prices, is quite incredible.