I work for a nonprofit. We absolutely don't expect everyone to respond to these types of "invitations" at all. Don't think of this as a personal invitation which requires an RSVP, think of it as an advertisement for an event and throw it away; similar to if the California Philharmonic sends you an "invitation" to buy season tickets to their performances. If you don't want to attend, don't respond at all. If you mark the box "I am unable to attend. Please accept my donation of (fill in the blank) to help the children of St. Jude" you will in all likelihood get a phone call from a fundraiser to verify what amount you intend to give and they will undoubtedly give you the hard sell of why they need your support. Also, if you respond, you also will probably remain on the list for future mailings. I know that for us, after 3-5* non-responses names are dropped off our mail list. If you are personal friends with someone at the organization, try asking him/her to please not send any future mailings to you.
*I actually don't know what the magic number is, but there is a point at which we give up because it costs us money to print and mail the piece and we only want to send to those we think might attend/give.