The entry of an order or decree of support for a spouse or for support and maintenance of a child under the provisions of this chapter or §§ 20-107.1 through 20-109 shall constitute a final judgment for any sum or sums in arrears. This order shall also include an amount for interest on the arrearage including from the date support is established or retroactively modified at the judgment interest rate as established by § 6.2-302 unless the obligee, in a writing submitted to the court, waives the collection of interest; and may include reasonable attorney fees if the total arrearage for support and maintenance, excluding interest, is equal to or greater than three months of support and maintenance.