What "their monthly service" are you referring to? A Roku has no inherent service/fee, meaning you can watch a lot of free stuff - but generally with boatloads of commercials. In this sense it's like a TV, only for internet instead of Over-The-Air.
However, if you want "premium" (Ad-free) services like Netflix then you pay for them. You can pay for them completely independently of Roku, or via Roku. Most here would suggest the former - ie: keep your arrangement with netflix as it was before and just log into your netflix account via the roku.
But it's a little confusing when you say: "I had Netflix prior to Roku and I still have Netflix." On the one hand, the answer is: of course you would - the things are independent. However, if you have netflix running on your Roku and you don't know which way you accomplished that (logging in to your netflix account vs setting up a new roku-based subscription), you could be in for confusion and possibly being billed twice for Netflix each month.
