Have been debating posting this because it is so simple that I figure it must have been posted in one of the many Kellogg's certificate threads but since it never dawned on me until the K-mart sale of three weeks ago I thought it might be of help to someone else who never thought of it.
Wal-Mart's policy is to match ANY competitor's price if it is in writing. So when K-Mart kept running out of stock I took their ad to WM, they matched it, and proceeded to buy the 157 boxes I needed all in one visit.