Item calculator

I’m new here so don’t know if I’m breaking any rules by requesting help as a first post. Please let me know if I do.

Ok so here’s my problem. I have to make a application for college…but our AS3 classes from last year never got any further than making movieclips.
I’m not a complete noob when it comes to simple coding, but here’s where I get stuck:
I made an application where you can add things to a cart which shows the total cost of the items.
And there a 3 different frames in which you can add things.
So I already made the output text area expand over these 3 frames.
But I’ve got no clue how and what code to use to add or delete the cost of items.

here’s a quick sketch of the setup:

1=name of one of the 3 different catagories (this being food, others are drinks and newspaper)
2=price and buttons to add/delete one piece (+price) to the total
3=output with total ammount.

Can anyone help me with this?