I tried solving the version of the problem in which codes are not necessarily unique, the grid an array of 36 squares in which each letter/number contained in any of today's specials has been crossed out, an item "appears" to be available iff all of its letters/numbers have been crossed out, and the customer picks one of the 250 items uniformly at random.
It seems that this is not the correct interpretation of the problem statement.
It seems that this is not the correct interpretation of the problem statement.