A quick answer might be this.  One might suggest, first, that one ought to be instrumentally coherent, because it is a necessary means to, or at least a necessary condition of, having the particular attitudes (or pattern) that one independently ought to have, or not having the particular attitudes (or pattern) that one independently ought not to have.  Suppose that one ought to intend to M, but does not.  Just as (P or Q) is a necessary condition of P, so too a necessary condition of intending to M is (either not to intend to E, or not to believe that one will E only if one intends now to M, or to intend to M).  Now suppose that instead of intending to M, one drops one’s intention to E.  While one hasn’t intended to M, which one ought to do, one has fulfilled a necessary means, or at least a necessary condition, of that.  Assume Ought Necessity of §2, perhaps slightly strengthened: that one ought to take any necessary means to, or at least fulfill any necessary condition of, what one ought.  In that sense, by dropping one’s intention to E, one has done something that one ought, and so satisfied a requirement.  Analogously, if one ought to post the letter, then, by similar reasoning, one ought (either to post the letter or to burn it).  So, if one burns the letter, one has done something that one ought, and so satisfied a requirement.