Instead of asking the public for a one-off donation (as what may have happened previously with door to door charity collectors or events such as telethons) collectors try and get people to sign up to a direct debit scheme where monies are deducted from their bank accounts each month.
Assuming you itemize, that payment can be written off as a charitable donation because some of it goes directly to research and the rest pays expenses you incur doing volunteer work, which is always deductible.