Now and again I get a message in my inbox asking me to check out someone's video but it's usually not my interest, so I just delete it (and I understand they're just trying to promote their channel, so it doesn't bother me at all). Though if someone sent me a video that was similar content to my channel, then there's a good chance I'd check them out. When you target the right audience with your message, most people don't seem to mind as long as you only do it rarely.
But I have another idea for you if you're not comfortable sending messages:
Have you signed up with Google AdWords? Before you think "it's too expensive", I'm not saying to actually buy ads. My idea is just to sign up and connect it to your YouTube channel. When you create an AdWords account they will want you to connect it to a YouTube channel so that you can promote your videos. Go as far as doing all that and having everything set up (you might have to set up credit card info, etc), and perhaps go as far as creating an ad for your video. But don't actually pay for anything. Just let it sit there.
AdWords will do a promotion now and again and offer things like a $100 voucher to put towards ads. They seem to send them out when they see that you're not using their service. I signed up with them and did a little promoting but the results weren't really great, so I stopped using it. Since I stopped using it, they've sent me two $100 vouchers over the past 18 months. The second one I got just recently, but I never actually cashed it in so it's expired. If it was still valid I could've given it to you lol
You may even be able to find additional vouchers on the net if you do a search.
I would not recommend sinking money into AdWords as your bang for buck is awful compared to the targeted marketing you can do with TubeToolbox. But if you're able to obtain a voucher somehow, then you might as well take advantage of it.
