Cost     | Cost ItemA (each)  | Cost ItemB (each)
-----------------------------------------------
$400     | 16c                | 10c
$800     | 12c                | 8c
$1200    | 8c                 | 4c

- Spend 2000 of ItemA
  Reduce by $320 (16c each), $80 remaining of first credit buy

$1280 overall remaining

- Spend 2000 of ItemB
  Reduce by $80 (10c each) 800 credits
  Reduce by $48 (4c each) 1200 credits