Yes, you're on the right track. Aggr allows you to group over a specific set of dimensions.
For your purpose, you may not need it. You could create a chart or table that uses postal code as the only dimension. Then create an expression for Sum(Sales_Amount) and you have sales by postal code.
If you want to create a table with Sales_Rep and postal code as dimensions, but have one of the fields be a postal code sales total, then you would use the Aggr function. Something like: aggr( sum(Sales_Amount), Sales_Postal_Code )