MacBook Pro Mid 2012 Memory

hallo,

i have macbook pro 13in mid 2012 [ core i5 ram 4Gb ] , but manufacture date is 2015 ,i took with app coconut battrey. i can upgrade my default memory 4GB and i took an apple site specification [ upgradeable to 8GB ] i see the other forum local and outside the other country can upgrade to 16GB, is it true ?

i can buy use the corsair 8gb x 2, i'm so dizzy because 1.35V and 1.5V .... which one will i use that ?

low voltage is better or its the same ?

and one question again.. why OWC the price is expensive ? i found in my local country store. because corsair is under price ?