You have to let them know. Write and call your representatives.

Politicians are quite worried about what the public want. If a bunch of constituents ask about this, the question may end up in an internal poll, and so on... Lobbying is an important source of funds for politicians, but it’s not everything.

Obvious recent example in the US; the dramatic shift in Democratic Party support for single-player as it became clear that that was something that constituents were extremely concerned with. Obviously that’s a big issue. But this can happen with smaller issues, too.

Right now, your representatives don’t know what’s important to you. Telling them will at worst do nothing, and may well help.

(Regarding a generic "you":)

If you've called your representatives, you gain my sympathy and concern.

If you've assumed ten minutes a month isn't worth it, these arguments are not persuasive.

And what impact does THAT have on anything exactly?

I thought of a thing that might have at least a little.

edit: I know I strode a little from the mandate part cause mandates may differ in relative importance for everyone.

Perhaps I'm not following your intention. I saw:

Person A: voting helps

You: How do they know why you're voting on this?

Person B: You have to tell them

You: But others spend more money and thus count for more

Me: That's an excuse unless you've actually tried it (and if you have, yeah, that sucks)

If I misread, my apologies - based on on your response it appears I have.

Its a good summary!

I didn't make the connection "these arguments are not persuasive" => "its an excuse" and rather read the part as "not persuaded by the argument" and so I tried to clarify.

(Didn't look so good did it:) Thanks for taking the time to clarify ergothus.