I use the +abc trick, and change it for each place I donate to. I've tracked some sharing, and at least it's easier to filter.
I used to do that but stopped. I've just updated my policy to no online donations whatsoever. If someone approaches me in person and tells me that's the only way to donate, and I want to give to the cause, I tell them to put their own email address there and forward me the receipt.
This past week someone tried to get me to donate online, and wouldn't accept no for an answer. I gave him cash, but he kept on insisting, eventually, he realized what a fool he made of himself, and wanted to give me back the cash. I refused telling him to give it to any cause he finds worthy, but he just left the money on the table and walked away. A day or two later someone approached me for the same campaign, but this time with a card reader in hand, I said "sorry, I don't give to this cause" (which is regretful because as a policy I try to never say no, the amounts might differ, but I try to never turn away anyone asking for tzedoko, especially if it's a yeshiva or anything connected to supporting, enhancing and/or spreading לימוד התורה).