Whether you can accept an invitation to an event or not, responding is very helpful to the host, and not responding may be inconvenient or even costly to her. You can communicate your intentions to the host in a variety of ways, and the host will undoubtedly appreciate your consideration.
In French: "Please Answer"
RSVP in French translates to "repondez, s'il vous plait" which means "answer, please." When an invitation contains this acronym, it is important to reply with an acceptance or your regrets. However, the phrase "regrets only" on an invitation implies that the host assumes you will attend the event unless you respond otherwise.
Your Response: Make It Fast
According to the Emily Post Institute, your reply should be as prompt as possible. Expressing your regrets without delay may afford the host the opportunity to invite someone else in your place.
Choosing the Method of Your RSVP
Allow the tone of the event, the host's preferences, and your relationship to the host guide the method of sending your RSVP. For example, a formal reply, written in the third person, would suit a formal wedding invitation with no reply card. If the bride is a personal friend, a brief handwritten note is appropriate. For less formal occasions, RSVPs may be expressed through email, over the phone or even via text message.