Buying add-on for EU in addition to using 12gb data allowance

Hi, 

I've searched for an answer but haven't found anything that specifically answers my question. 

My daughter is going to Italy tomorrow and so will have her 12gb data allowance to use. As she is staying in a place with no Wi-Fi I know that this will be used therefore I intend to buy an add-on today before she leaves.

My question is, does the 12gb "free" allowance gets used first and then auto switch to the add-on package I've purchased? And just a supplementary question; if the add-on gets used can I just buy another one and this will immediately be available for her to use? 

Many thanks