Sum of Invoice Subtotal Amount

Hi, I'm trying to create a simple suitescript search that will sum up the subtotal field of invoices for a specific customer over a specific date range.

However I can't seem to figure out what the field ID is for 'Subtotal'.

Instead of using Subtotal I tried getting the amount, shipping amount and tax amount so I could calculate the subtotal but the values that get returned don't make any sense to me.  They are much higher than the values on the invoice.

My simple script is below.  Please advise on the best approach.

Thanks,
Blair.

var invFilters = new Array();