You will find the term “Merchant’s Thumb” Principle in note 3 to Chapter 10 of Metaphysics, on p. 165. The Principle itself is the indented material on p. 152. We’ll call this MTP-2 (the second version of the Merchant’s Thumb Principle), since there was a shorter version of the Principle in the first edition of Metaphysics. Here’s MTP-1 (the Merchant’s Thumb Principle as it appeared in the first edition of Metaphysics):

Suppose there is a certain fact that has no known explanation; suppose that one can think of a possible explanation of that fact, an explanation that (if only it were true) would be a very good explanation; then it is wrong to say that that event stands in no more need of an explanation than an otherwise similar event for which no such explanation is available.

Note that it was MTP-1 (the one indented directly above) that I used the “splitting the pizza” case against. Consider: Does that case, or any other example, work against MTP-2 (the principle indented on p. 152 of the second edition of Metaphysics)?