It might be an honest mistake. Send him an e-mail explaining that his auction listing specified $5 shipping. If it isn't a mistake and he still wants $20 for shipping, send him another e-mail telling him to go pound sand down a rathole and leave him neg feedback! He'll probably neg you back but who do you think it will hurt worse??? You with a feedback score of 2364 or him with a feedback score of 33???

Cheers!
Bill