Being on a responsible seat of tax collection department, if a taxpayer offers you $20 in return of your good services, the appropriate action would be NOT to accept the bill. Say him NO in a polite manner and tell him that this is his job and he is already paid by the company for his quality services, so he cannot accept the money.
This will make the taxpayer even happier with the services.
So saying him No, would be the appropriate action.