Police would likely shy away as they are under such scrutiny. Postal employees are limited to $50/yr in tips. With most fire departments being volunteer, occasionally tips or donations rather are offered. These generally would go to the department rather than individual firefighter.
My stepfather was a postal carrier and was adamant that postal carriers need to refuse cash gifts, but could accept small gifts of other sorts around Christmas.
I assumed that he was saying what the postal code says, but perhaps it was more a local culture thing.
https://about.usps.com/postal-bulletin/2012/pb22349/html/cov...