Although I cannot find the URL, I recall reading somewhere that, on average, Amazon prices were 9% lower than big boxes' (BBY, et al). not clear whether that was 'bottom line' price comparison (in which case AZN's prices would not include tax whereas brick & mortars' would. Now that AZN agreed to collect sales taxes for a number of states that delta will shrink.

But, I do believe that convenience of easy ordering (for lazy ppl like me), is bigger differentiator than a couple of percentage points on pricing.