Newbie needs advice on a memory upgrade

So, I'm running a MacBook Air, 3542, with an i3 processor and 4GB of RAM at the moment. It's running macOS, and honestly, it gets pretty irritatingly slow sometimes, so I'm thinking about upgrading the RAM.
It only has one slot, and it requires DDR3L memory specifically.
Specifically, I had my eye on this one https://www.ebay.co.uk/itm/16GB-RAM-...sAAOSwceZcjjTI

Or maybe it would be better to just go with a new 8GB stick?

You can't fit 16GB in there, just 8. So you'll need one 8GB stick.

Where are you located?
The stuff you linked is overpriced and shipping out of Lithuania, so why not just buy something new?

I'm getting a new one

I don't really get the difference between this one ---> https://www.amazon.co.uk/Kingston-KV...gateway&sr=8-4
and this one --->https://www.amazon.co.uk/Kingston-KC...gateway&sr=8-6

Which one actually fits in my laptop?

And why on earth is Dell memory so expensive? https://www.amazon.co.uk/DELL-8GB-DD...5&sr=8-1-fkmr1

They’re overpriced if you happen to stumble upon some random certified memory; nobody in their right mind is buying that.
There isn't much of a difference between those two anyway.
I'm guessing you're over in the States?