Afaik, PC download= they give u the key
PC instant access= U sumhow link ur origin account to amazon and they directly add the game to it
I checked their help section...PC Download means you download it from amazon using their own download client similar to origin.
Contacted Amazon and asked this
'I want to purchase a digital version of the game so that I can download it after purchase. I wanted to buy Battlefield 4 pc download but I want it to be attached to my origin account. So which option should I select 1. PC download or 2. PC instant access so that it can be attached to my Origin account??? '
and they replied --->
'Amazon Games & Software Downloads can only be purchased by customers connecting to the Internet from a location in the United States (specifically the 48 contiguous states, the District of Columbia, Alaska, Hawaii, Guam, and the U.S. Virgin Islands).
A U.S. credit card with a U.S. billing address must be used to purchase Game/Software Downloads. And, once your 1-Click Payment Method is updated with the said requirement, you'll be able to purchase the digital order of Game. '