Hi Loftyy, if you are in ecommerce site, adsense is the most important to get targeted traffic and surely you will get more sales. Advertisement and promotion are really important. In my opinion, seo in ecommerce site is important for short term. so you dont have to buy seo package till 5 years continuously just 1 till 3 blast for your site. link building is a good one.
BTW you paid $100 dollars monthly ?
you can PM me maybe i can help you for advice more