Apple are telling me that it is possible to add extra RAM from 4GB to 8GB but an independent computer company says this is not possible, who is right?

Recently bought a Macbook Air with 4GB RAM, wasn't asked at the time if I wanted more and at the time didn't think I would need more, that is until I wanted to download 'Adobe Illustrator' (requirements recommend 8GB).

Rang tech help and they said it is possible but Apple don't do it in their stores but a 3rd party could do it. Found a 3rd party (name given by someone at the Manchester apple store - can't remember the company) but they said it is not possible to retro fit RAM to a MacBook Air as there are no loose components in the motherboard and I should have been asked at the time of purchase if I needed extra RAM.

Rang tech help again yesterday and the guy sent me a link of specs for my laptop and insisted it is possible but when I looked carefully at the specs (OSX mountain lion) it does say '8GB memory upgrade' but under the heading 'Configure your MacBook Air to order with these options' - which I interpret as you can only have the extra memory BEFORE you buy.

Does Anyone know if I can add more RAM?

I am also about to chuck my magic mouse out of the window as it is constantly losing its connection and telling me the batteries are low (replaced them 3 times already), rang tech about half a dozen times about this, gave up and plugged in my old usb mouse.

Would be grateful if anyone has any answers to my question.

Many thanks😕