So, in other words, the position of the cell containing the sum on the secondary sheet will change, right? In that case, you would need to adjust the formula on the primary sheet.
You could leave some empty rows in the secondary sheet, above the formula, to allow for adding of new items without necessitating an adjustment of the formula on the primary sheet each time a new item is added to the secondary sheet.
Another solution would be to convert this whole application to a Microsoft Access database.