Politely decline an offer that I sort-of accepted
Dear [name],
  
  Thank you for your reply to my email of December [xx]. Please note that in that email I asked what documentation I need to send in the event that I decide to accept the offer to join your graduate program, but did not yet indicate a definite wish to accept the offer. In the meantime, I have considered the matter some more and decided to pursue other opportunities elsewhere. Thank you again for the generous offer, for which I am  grateful.
  
  Sincerely,
  
  [your name]