Commentary for Menachot 81:11

Do you punish [even for a positive commandment: of this type]. For a man is not obligated to purchase a cloak that requires [tsitsit], if [he] does not [have one]. But concerning a full positive commandment (in which the Torah obligates us to do something), even the earthly court may strike him until he dies.

At that time - at the time when the corpse is dressed.

We certainly affix - we put tzitzit [on the shroud] because were they not attached, it would be like mocking the dead. As we have learned, the dead are exempt from mitzvot.