Saving data to order details from checkout script

[Jiri_V]

Hi to all,
I would be very grateful for help or at least pointing in the right direction in the matter described below. I am still a noob in this.
In our store, we want to implement a functionality to let users choose a pick-up point from provided map at checkout (when they select certain shipping method).
We are hoping to achieve this using the checkout script placed to the checkout page in Shopify settings.
Where we are stuck is the way to store the user's selection and assign it to the order (possibly as a note or attribute).
Is this even possible to achieve using just the script? If so, how? If not, what are our options to achieve this? Would we need to develop a separate extension for this?
Thank you for any replies.