Rambam on Pirkei Avos 3:15 suggests that it is better to give a certain amount of tzedakah each day over several days rather than giving the equivalent total all at once, because each act of giving amounts to a unique mitzvah.
In this regard, what if someone has a tzedakah box in their home, and after contributing to it for a time, decides to collect the contents and deliver it to a needy person? Does this constitute a single act of tzedakah, since the money remained in the donor's control until the end, or does each act of designating funds for tzedakah equate to the mitzvah of giving charity?