If one gives Tzedaka through a will -- so it is meant to be given after death -- does that person get the Mitzvah of Tzedaka?
The reason I would think "no" is because it is difficult to understand how someone can get a Mitzvah after death, and what exactly that would mean. Mitzvos are meant to be done in this world.
The Kitzur Shulchan Aruch writes in סימן קצג - הלכות בקור חולים
סעיף ו'
אֵין לִתֵּן נְכָסָיו בְּמַתָּנָה לַאֲחֵרִים וַאֲפִלּוּ לִצְדָקָה וּלְהַנִּיחַ אֶת הַיּוֹרְשִׁים בְּלֹא כְלוּם. וְכָל הָעוֹשֶׂה כֵּן, אֵין רוּחַ חֲכָמִים נוֹחָה הֵימֶנּוּ, וַאֲפִלּוּ אֵין הַיּוֹרְשִׁים נוֹהֲגִין כַּשּׁוּרָה. אֲבָל אִם מַנִּיחַ גַּם לַיּוֹרְשִׁים דָּבָר הַמַּסְפִּיק לָהֶם, מֻתָּר (וְעַיֵּן לְעֵיל סִימָן לד סָעִיף ד).
One should not give away one's property - even to charity - and leave one's heirs penniless.
However, one may do so, if the inheritance is sufficient to provide all heirs with suffient funds.
The Kitzur then refers one to סימן לד - הלכות צדקה where he writes:
סעיף ד'
כַּמָּה יִתֵּן הָאָדָם צְדָקָה. שָׁנָה רִאשׁוֹנָה הַמַּעֲשֵׂר מִן הַקֶּרֶן. מִכָּאן וָאֵילַךְ יִתֵּן מַעֲשֵׂר מִן הָרֶוַח שֶׁהִרְוִיחַ כָּל שָׁנָה, [חוּץ מִצָּרְכֵי בֵּיתוֹ], זוֹ הִיא מִדָּה בֵּינוֹנִית. וּמִצְוָה מִן הַמֻּבְחָר שֶׁיִּתֵּן חֹמֶשׁ שָׁנָה רִאשׁוֹנָה מִן הַקֶּרֶן, וְאַחַר כָּךְ כָּל שָׁנָה חֹמֶשׁ מִן הָרֶוַח. וְאַל יְבַזְבֵּז אָדָם יוֹתֵר מִן הַחֹמֶשׁ, כְּדֵי שֶׁלֹּא יִצְטָרֵךְ הוּא אַחַר כָּךְ לַבְּרִיּוֹת. וְדַוְקָא כָּל יְמֵי חַיָּיו. אֲבָל בִּשְׁעַת מוֹת, יָכוֹל אָדָם לִתֵּן עַד שְׁלִישׁ רְכוּשׁוֹ צְדָקָה.
The correct way to give charity: A one-time 10% or 20% of one's capital and then of the yearly profits.
One should never give away more than a fifth (or a quarter) so as not be become dependant on others, as a result.
However, at the time of one's death one can give up to a third of one's property.
Point being, we see that it's permissible to give charity in a will. Since it's an action you did (or caused to happen) when you were alive, you get the credit.
There's a tosfos out there (don't recall where, heard it on a shiur) that there were donations to the Mishkan of gold, silver, and copper. Tosfos comments: a "gold-level donation" is when a young, healthy person gives tzedaka. A "silver-level" donation is when they're older or sick. And a "copper-level" donation is what they leave in their will. Maybe not quite as super-duper, but still a wonderful thing.
A person can be judged on what they set into motion in their lifetime, even if those mechanics played out after they died. (Just as King David got credit for putting every thing into place for building the First Temple, though he didn't complete it himself.) Setting up a will would count. So would buying a life insurance policy to make sure one's loved ones won't be stuck in a catastrophe afterwards.
