1) You Must Reply to an Invitation
Foreign custom is much more strict than Chinese custom in the matter of replying to invitations. When you receive an invitation you should answer it immediately, saying definitely whether you are able to accept it or not.
2) Written or Verbal Reply
If the invitation is given by word of mouth, in conversation or at a chance meeting, you should answer at once whether you can come or not. If you cannot give an answer at that time, you may say, "May I let you know this evening?" Or some such words. 商务礼仪―― Invitation