Not trying to give the OP a hard time, but I find it funny when people say they got the rMBP because they "needed" the screen/resolution. How can you know you need something if something like it hasn't been available before?
Needs change my friend, When I last bought a macbook pro, I was using a Nikon D700 as my main body, that was a 12MP camera. I bought the hi-res non-glossy upgrade display for that and loved it. Now I'm using a D800 which is a 36MP body. The images are much bigger, therefore I 'need' a higher resolution display. Of course this is pretty low on Maslow's Hierarchy, but then so are all computer products. There are many things I need that haven't been invented or released to the public yet. Time travel, hover cars, teleportation, etc. etc. Probably more mundane things too, like a faster MBP with even higher resolution
I do research all my tech purchases, and I knew before getting this computer it was what I needed (/ wanted), but not because of it's upgrade potential. As a tax deductible cost it is cheap relative to the income it generates, so that wasn't a concern. I didn't research the upgrade potential specifically, because I knew until now there is not much you can do to a laptop, beside RAM & HDD. I'm glad none of you were caught out by this, but you are obviously active MacRumors forum users. Just because you don't monitor these forums carefully enough, doesn't mean that you a) don't know how to use a computer, and b) don't need to... sometimes it means you are busy. I saw there was a 16gb upgrade option, so made the wrong assumption that you could upgrade that at a later date. I'm not scared of taking a few screws out, but solder!? No way.
Wait, so I'm thoroughly confused?
What memory did you buy? The device comes with DDR3L ram. Even if you suspected there was a sodimm, where do you find DDR3L in a sodimm?
So again, what memory did you buy that you thought would work?
I'm not technically minded enough to know about the voltage of RAM etc, but I did try to check the RAM would work. Obviously I made a mistake, but I don't think it was a mistake no-one else can make. I used the online hardware checker at Crucial, checked for my machine (MBP 2.6 i7 15" DDR3 Mid-2012), saw it was available for £60, which is £100 less than the apple upgrade, it said it was guaranteed compatible, and was DDR3-1600. If you look at the about this mac screen below you will see it is shown as DDR3, not DDR3L.
It didn't jump out at this point as incompatible to a non-expert in RAM. If the RAM is DDR3L as you say, it's not shown as such on about this mac:
BANK 0/DIMM0:
Size: 4 GB
Type: DDR3
Speed: 1600 MHz
I'm really not into the very technical level of components, I use computers as a tool which is the main reason I use a mac 'it just works' (... well used to). It might be helpful for Apple to label the RAM in about this mac as non-upgradeable, that would have helped.
Although I don't mind spending/investing money in hardware to improve my work, I don't like to throw it away either, and that is why I thought the RAM upgrade could wait as the price of memory tends to fall with time. Luckily it was already cheap, so I caught this problem in time, others won't... that's my point.
Yes it is the OPs fault. When you buy the MacBook Pro, it says it is not upgradeable as you're going through the configuration options.
That is true, but I didn't get mine online, I got it in the store where I didn't see any warning. I accept I should have known, but I didn't. I think others may not too, and will be equally disappointed.
PS. I realise 'retina' is not specified, and that is is key here... but it's definitely not clear. About this mac should state
MacBook Pro Retina, Mid 2012.
Not
Macbook Pro Retina, Mid 2012
if it is an entirely different line for RAM purposes.