(a) Payments made by a taxpayer, other than installment payments of a tax liability which is required to be paid in installments, shall be allocated to the taxpayer's outstanding liabilities as follows:

1. (1) First, to any assessment which has become final;
2. (2) Next, to any fee which has not yet been assessed;
3. (3) Next, to any tax or related charge which has not yet been assessed;
4. (4) Finally, to any assessment which has not yet become final.