Im assuming u mean like chessed when u say mitzva. I dont think youre supposed to allow the person to take advantage of your kindenesss and let them walk all over you and abuse you. Also cud be you cant do a chessed for someone that doesnt appreciate it, (i assume abusing you especially.)
And protecting yourself is important as well. We have to be honest about where we are holding by and not go overboard with our avodah into situations that are too much and over the line, imbalance. Im not sure i know what the deal is with the tzedaka box but sounds weird.