You create a Summary field for Total Charges that will show sum total in PREVIEW mode when sorted by the sort field named when you create the Summary Field (TotalChargesSum?)
Check FMP Help for Summary Field.
The summary field David describes will also be visible in Browse mode. THere is no sort field to specify for a summary field.
I think he's referring to placing this field on a sub-summary part to show sub totals. In that case you have to specify a sort field for the layout part and the subtotal will not appear in browse mode for versions of FMP prior to FMP 10. (Sub-summaries are visible in Browse mode if you use FMP 10.)
If all you want to do is pull up a group of records with a find and see the total at the bottom, simply define your summary field and place it in the footer of your layout.