CrustySockBro can't even get this right!
As others have said, SAE is a self (stamped) addressed envelope - before t'internweb it was common practice to enclose one when asking someone for something so they didn't have to pay postage
eg "Dear Ioan, please can I have your autograph? I enclose a SAE. Love Tone"!
What he means is "send your answers on a postcard"...
Frequently used when asking a rhetorical question.
eg "why is TinyTone such a wank bucket"...cue British Tattlers falling about laughing and demanding answers on a postcard please!
What a dumb arse he is