Edits can be made here: http://code.stypi.com/aexmachina/order-states (but Stypi doesn't support YAML syntax)

order-states.yaml

root:
  initialState: unpaid
  actions:
    - recomputeState() # throws Exception if 0 > paid || paid > total
      # see comments on states below
    - addItem(item) # then recomputeState
    - payment(amount) # then recomputeState
    - refund(amount[, transaction]) # then recomputeState
      # if refunding line items then refund amount may be computed from
      # adjustedTotal, and do we care what items a manual discount
      # was applied to?
    - undo(transaction) # then recomputeState
      # this may require us to confirm that the parent is the latest
      # transaction to avoid race conditions
  unpaid: # total of transactions is 0
  partialPaid: # total of transactions is < order total
  paid: # total of transactions >= order total

"order model":
  attrs:
    parked
  computed:
    hasRefunds
    total
    totalPaid

"lineitem model":
  attrs:
    placed
    adjustedTotal
      # this may be a solution for refunding individual items
      # need to find out whether refunds should be user-controlled
  "hasMany transactions":
    # this does not indicate whether a lineitem is partially/fully
    # paid or refunded - this would require a state machine (which
    # is conveniently the same as the one above), and is outside scope

"transaction model":
  attrs:
    references (was parent)