The other day I was stopped on the sidewalk by someone who told me they worked for a large environmental non profit that helps save endangered species. He asked if I would subscribe to help. I offered to make a donation and was told they I can't do a one time donation and they "really need subscribers". He even told me I could just cancel after the first payment. Taking advantage of someone being forgetful is sleazy.
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[0] - I'm bitter about this wrt. banks in Poland. I should be able to view all recurring payments that pull money from my account in a single place in a web interface, and cancel them from there. Alas, the banks are happy to send out my money, but offer no way for me to see a list of recurring payments.