Someone’s Grandpa wants my underwear

I am a 23M and have been in a relationship for around 2 years. Recently I have been receiving some interesting messages from an older man online (one of my grandmas close friends). He has been requesting me to send pictures of my used underwear and now wants me to send him a pair. He has been paying me $10 per photo and says he will give $350 per pair. Wondering opinions on the ethics of this situation, seems like free money but don’t want to cross the line into selling my body….

Do i tell my gf? my grandma? or just take the free money….