If you feel that the value reported in the Average Invoice Value in the Sales section is incorrect, you can validate the value in the field by doing the following:
Make a note of the total sales amount realized from the sale of all items in the Daily Summary report under the Sales section (Reports tab > Daily Reports > General > Daily Summary > Sales section > TOTAL field).
Make a note of the number of the invoices from the Sales Reports - Invoices report (Reports tab > Finance > Sales > Invoices (General)).
Divide the total sales amount by the number of invoices. This is the average invoice value.