mem upgrade for acer 6360S

my gal has a acer aspire #6360S & she wants to get a memory upgrade to 128.
We tried crusin/surfing thru a couple of online sellers and are totally ‘bedazzled’
…Just how “case sensitive” are these things-None of them say Acer ‘econo replacement’like at the car parts store(just a pun attempt)..acer wants 60-$150+….we see mem advertised for 20-$45,at the online places & want to save the cash- but do not want to hurt the PC..
…Are they plug & Play
…if I just had a clue what to get I’dorder the darn thing & have it shipped…
so here we are