Yeah, they get lots of gifts. Congressman and senators do a lot more than just passing legislation. Even a slime ball like Murtha probably helped someone out with getting a scholarship/red tape/charity event. People constantly send in gifts.
If they are truly getting tons of gifts from constituents, the $20 is low. Even if they don't get a lot $20 is too low. I would say $100 is a better limit, and that would be at the bottom end.
And its not like they are the ones who actually fill out the forms. I guarantee they have staff do it.