"The costs to send out those letters is a large sum, and if they reduced the quantity, they would actually be receiving more money."
I've dealt with this, and related, issues before.
I worked for a number of years in the direct mail industry with a major financial institution (Navy Federal Credit Union), and was for a number of years before that associate editor of American Rifleman magazine.
Direct mail is used as a fund raising vehicle for one reason and one reason alone -- it is, by far, the single most effective fund raising technique there is. Nothing else comes even remotely close.
No organization would continue to send out direct mail solicitations if they failed to bring in funds.
Often, a DMS brings in funds that FAR exceed total production costs, while the break even point, the point at which soliciatations have paid for the production costs, can often be met by a response rate as little as 0.25 percent. That means that even if 99.75% of the people on the mailing list throw the solicitation in the garbage, the solicitation is still not a failure.
For organizations such as NRA, ones that have a solid, cohesive message on a subject, response rates can easily exceed 10%, especially if there is a particular, pressing issue (such as an election). Average donation levels also tend to be much higher for organizations such as NRA, which can push that break even response rate even lower.
Simply put, to those who are unfamiliar with the concepts behind direct mail marketing, it may look as if NRA is wasting huge quantities of money.
Trust me, folks, they're not. They're bringing in a lot of money, even if you only participate once or twice a year.