RAM Upgrade for My Dell Inspiron 15 5505


I have just bought an Inspiron 15 5505 4700U 8GB 512SSD, and I was wondering how to go about upgrading the ram.

I looked at Dells website and they are charging $138.55 for an 8GB SODIMM stick.

I am wondering what other options do I have?
Do other places sell the same stick at a cheaper price.
If not would I need to buy two new 8GB SODIMM sticks of RAM so that they are matching.

Thanks for any knowledge shared as I don't know a lot about laptops.

Also also, does anyone know a good laptop case that would fit this laptop?