In the Lost Item Fee Policy, the aged to lost settings are written such that the patron is supposed to be billed 'X intervals' after the item is aged to lost. If a library bills 1 week after an item is aged to lost, and the item was aged to lost on September 1, it should be billed on September 8 (or as soon as the batch aged to lost billing process runs after that date).
After talking to Bohdan, I learned that...
Currently the aged to lost billed date is being calculated as:
dateLostItemShouldBeBilled = loanDueDate + agedToLostAfterOverduePeriod + patronBilledAfterAgedToLost.
When it should be calculated as:
dateLostItemShouldBeBilled = agedToLostDate + patronBilledAfterAgedToLost