When you receive a request for a financial contribution from a political party or individual politician, write to them in your own words why you have decided to decline and enclose the letter in the pre-paid postage envelope.
If enough people do this, Congress will get the message.
That’s actually completely pointless. Either 1, they’ve contracted with a firm that does nothing but send mailings and receive money who’ll just throw it away without comment, or 2, it was received by a minimum wage employee who’s been hired to put a check in a box next to your name on a computer so some other minimum wage staffer can send you a thank you card, or it gets thrown away with no check in the box. They don’t even bother keeping tracking of who didn’t donate.