Except for expenses the household elects to average, the expense is deducted in the month it is billed or otherwise becomes due, regardless of when the household intends to pay the expense. For example, rent which is due each month is included in the household's monthly shelter deduction even if the household does not pay the rent each month. A particular expense may be deducted only once. Amounts carried forward from past billing periods are not deductible in a subsequent month even if included in the most recent billing or actually paid by the household in the most recent billing period.