How do I sum a ROW of numbers in Access and avoid NULL propagation

Question

Problem is Null Propagation. I am trying to sum all of the items sold (columns) for each Order ID/row. At the end of each Order ID/row, there is a Total field (column) in which I want to display the total $ amount for all the items sold for that Order ID/row. The problem is that none of the order ID rows contain ALL of the items, so I run into null propagation, whereby the Total field for each order row displays blank (null). I don't understand the Access Help discussion of this. They talk about the NZ (null zero) expression, which converts null to zero in order to avoid null propagation, but I don't understand what I'm supposed to do for syntax. Can someone type an example of the proper syntax to use in Design View to calculate the sum of several numeric columns, any of which could be null on a given row?