When I worked for the federal government, every year around this time, they ran the Combined Federal Campaign (CFC) – https://cfcgiving.opm.gov/welcome sort of a federal version of the United Way, but administered by different groups at the local level. You could designate between one to five charities to receive your contributions. If you were in the military, you could also donate to the campaign.
I liked many things about this campaign as every year you could change charities, and how much was deducted each pay period. I generally went with five charities, divided into different categories. I tended to change charities every year and I often looked for nonprofits that weren’t the bigger ones as smaller nonprofits tended to get less support from the campaign.
Last year they expanded it to include federal retirees or survivors who receive an annuity from OPM. In the past, you had to be an employee to easily contribute.