Checkout throws "deadlock detected" under load

• During peak traffic, some checkouts fail with SQLSTATE 40P01 deadlock detected.
• The same code works fine under light load.

-- deadlock report (postgres log):
Process A waits for ShareLock on transaction ... blocked by process B.
Process B waits for ShareLock on transaction ... blocked by process A.
  A: UPDATE inventory SET qty = qty - 1 WHERE product_id = 5;  then WHERE product_id = 9;
  B: UPDATE inventory SET qty = qty - 1 WHERE product_id = 9;  then WHERE product_id = 5;