Lol.. That is the drawback to doing good deeds. The "human greed" factor can kick in. They see you doing a good deed and now they want you to do it for there organization as well(draining). I noticed this years ago when we have donated money for some charities. Alot of them come back every year asking for more. My attitude with that was to just ignore them and to donate when we can. Because of these over aggressive sales people once again who look to get you to give, give and give some more that's the greed factor. They work on your guilt as well. Personally I just block them out and give when I can.
Also over the years I looked into certain groups to see what % of your donation actually goes to the ones in need. The surprising factor is some are less then 5% of your donation goes to where you intended it to go. The other 95% goes to the "fat cats" wallets such as paying the chair millions and his BODs hundreds of thousands. When we find that out we no longer if ever donate to those groups.
Charity is complex. In our Vegas event because I know a couple of people who are directly involved with certain charities we were able to get our donation thru 100% which is rare.