You can not upgrade the memory in these laptops. You must order them directly from Apple.com with upgraded memory. The memory is soldered to the motherboard. These are not like past mac laptops where you just had a panel you unscrewed and upgraded memory or HDD.
The model I have listed under ttNB has 16GB of RAM. Check the descriptions well. Amazon did not have the exact model of my configuration. Everything is identical except for added extras, including the RAM.