There's only room for one call to action in a fundraising direct mail envelope.
One thing you want your donor to do.
If you're sending your donors an appeal for funds, don't try to save on postage by slipping in something that is not about that appeal for funds.
Thinking about putting your newsletter in with the appeal? Don't. You'll get less response to the appeal or the newsletter.
Looking for planned giving leads? That's smart. But don't do it by putting a planned giving flyer in your appeal. That will cost you both in leads and in immediate revenue.
Hoping to get donors to come to an event, or volunteer, or write to City Hall? Great. Just don't double-up one of those calls to action with your fundraising call to action.
There's only room for one action in that envelope.
But there's lot of room for relevant, interesting things that point in the same direction. In fact, the more the merrier (up to a point).