Hello,
Actually there is couple option which you can use. I am going to recommended something for you based on my small experience. The best two render farm for me is RenderShot and AlphaRender.
It’s very simple to use, you just save file as package and then you upload it on their website, you got also option integrated on the website to choose quality and settings, but Always make sure you save render settings into your package file.
Alpha Render actually give you 30USD credit for free - this credit is with watermark, but there is simple solution how to delete watermark from output, you just deposit another for example 10 buck and now you got 40USD credit without watermark since you made deposit.
RenderShot in my experience little bit more pricey than AlphaRender, but not to much, just couple percent. But they got better customer service I would say, the reply when there is problem is much more quicker.
You can also you calculator on both websites to calculate how much is gonna cost your render, there is always something like 15percent more what calculator show, so for example if calculator show you, your project is gonna cost to render 10USD, more likely it’s gonna cost you 11-12USD. It’s not a big difference but with larger project this difference can make ‘‘difference’’ 
My recommendation - Alpha Render, if you are going to render something first time, and you want advantage at the beginning with that free credit, this credit can actually help you a lot with rendering.
But how they manage to do it? I have no Idea, I think they are using scripts or something, cause final renders are always uploaded on DropBox, to me it looks like all websites got same script, or similar maybe, cause that process of downloading final files are always same, and also there is lot of similarities.