循环后直接合计即可
……
For i As integer = 0 To Names.Count -1
rt.Cells(i+1,0).Text= Names(i)
rt.Cells(i+1,1).Text = Tables("订单").DataTable.Compute("Count(日期)","产品='" & Names(i) & "'")
rt.Cells(i+1,2).Text = Tables("订单").DataTable.Compute("Sum(数量)","产品='" & Names(i) & "'")
rt.Cells(i+1,3).Text = Tables("订单").DataTable.Compute("Sum(金额)","产品='" & Names(i) & "'")
Next
rt.Cells(Names.Count+1,2).Text = Tables("订单").DataTable.Compute("Sum(数量)","")
rt.Cells(Names.Count+1,3).Text = Tables("订单").DataTable.Compute("Sum(金额)","")