SSRS How to sum expressions

I'm new to SSRS.  I am using an expression to calculate an amount at the group level.  I can get that to work.  But what I can't get to work is how to then sum that expression to use at the higher group level.

The main group break of this report is by location.  But there is another group break of Invoice.  For each Invoice, I need to determine the amount of County Sales Tax due.  County Sales Tax = 2% up to $2500.  I can get this to work.  I can't figure out how to Sum the total County Sales Tax Due.  This is an Expression field.  So Sum won't work.  Below is an example of my data.  And how I want the report to look.

Data
Location               InvoiceNumber LineItem    Amount
AA                       1                     1             $500
AA                       1                     2             $1500
AA                       2                     1             $5000
BB                       3                     1             $10000  

Report
Location     InvoiceNumber    SubjectToTax    TaxAmount
AA             1                        $2000              $40
AA             2                        $2500              $50
                                            Total Tax AA    $90
BB             3                       $2500               $50
                                            Total Tax BB    $50
                                            Total Tax ALL   $140

I've added Custom Code to Sum the Calculated Tax Amount.  This almost works.  But when the report is first rendered, the Tax Amount is 0.  It doesn't get filled in until I click on the plus sign to expand each group section.

I've seen a lot of discussions about this on the web.  But I haven't found one that works or a newbie like me can understand.

Thanks for your help