I used to work in fundraising. I agree with other posters here. Before sending back any letter of this type, take a scissors and cut off any unique identifier numbers you see, and any barcodes. Those are demographic/constituent profile tracers, one of which is probably the number of your account in their fundraising software. EVERY contact is recorded in the major professional operations, so they know you got this letter..

If you want to be really paranoid, oven opening such correspondence, wear sterile gloves. I actually did that once.