To your point, you aren't Amazon's customer, the insurance companies are. They might be able to use their size as leverage for better prices from manufacturers

To your point, you aren't Amazon's customer, the insurance companies are. They might be able to use their size as leverage for better prices from manufacturers and compensation from insurance companies. There may also be synergies with Amazon's business lines because volume is a big factor in fulfillment costs. All of these factors might give them an edge over other online or brick and mortar pharmacies.