Ask HN: Cheap Internet marketing for bootstrapped Shopify app startup

I've just created a new app for Shopify. What I really want to find out is if there is a cheap internet marketing company that can do the marketing for me. I think it basically needs forum posting etc. I'm not trying to promote my app here, I just need some marketing advice on how to get some users!

My app has some competitors but also has some real differences that will add value to eCommerce type businesses and is the only app that provides those features.

Any marketing advice would be fantastic, but I really would prefer to pay someone to do it for me.