Going through a 3rd party would only result in them sending it to the same email address you could have in the first place, or possibly the wrong one, while the 3rd party gets some info about you and where to send the political spam in most cases. It won't give your message any extra power.
A simple no doesn't need "extra power", and as far as spam goes ... That's why I was/am inquiring about the site in questions reputability. Time is also a factor...
In fact, it would probably have a better chance of being read (and possibly responded to) if you sent it yourself. You will at least know it is going to the correct representative. (but don't count on the congressman himself doing it...probably will be handled by one of his aides.)
Given that most of congress is to busy, drunk, or stupid to use Email (or most technologies (Twitter...) properly)...It will assuredly be handled by an aid of some kind. In which case critical thinking skills are best left out of the requirements list. Better to repeatedly flash the same (exact) message in front of them for the purpose of it hopefully eventually sinking in. Just like training a dog; somewhere between to 1st and 5,000th time a command is repeated they catch on ... Trying to reason with them ($$$$$$$$) is simply foolish.
This is a big part of why dictators of old had to be lashed to a post and then set on fire before they realized that the people
weren't "happy" with them (e.g. Not everybody
If everyone handles it themselves, then all of congress will receive messages from the citizens they represent, rather than a single congressman or some general info email address that nobody pays attention to.
If everyone handles it themselves, there will be a bunch of different messages, worded a bunch of different was, many of which will detract from or completely obscure the point. And if the site in question has the address wrong, or is failing to send said message to all of the necessary/relevant parties... Well, that would be a part of the reputation thing I was inquiring about.