Insert a multiple selection into a table

I'm currently attempting to do something way over my head in Access 2003. The relevent tables are:

products
prodid (PK)
name
description
price

shopping_basket
basketid (PK)
custid
prodid

The listbox on the form runs from a query that gets just the id and name of each product from the product table. What I want is for the user to select one or more products from a listbox on a form and, along with the customer id, be inserted into the shopping basket table.

What I'm trying to do is concatenate the string so each selected product id is put into one row in the shopping basket table. Ie, if product id 1 and 3 are selected, the string 1,3 is inserted into the table.

One way I've thought to do this is to use a loop that put each selected product into a string, which is what is inserted into the table.

Or am I going completely the wrong way?

would be better if you created one table more, which will contain the choosed products:

shopping_basket
basketid (PK)
custid

shopping_basket_products
basketid (FK)
prodid (FK)
Quantity

and then have product 1 and 3 at one row each.