If you are looking for quality (and you should be), I would stay with what you have for now and rely upon getting as close as possible to the subject.
Something to consider is one of Canon's best telephoto values, the EF 400 f5.6. I bought one when I bought my 1D 2 back in 2005 and even though I have the better EF 400 f2.8 now I have held onto the much lower cost f5.6 version because it is nice for hiking and it produces very nice results. Although not cheap compared to the low cost Canon glass, it is a really good value for a Canon L series telephoto prime. I shot a lot of sports and wildlife with it and it does work well with the Canon 1.4X extender and continues to provide good quality as a 560mm f8 combination. If necessary to get this into a decent price range, consider a good used unit from KEH or a well known used dealer. Image quality is extremely good with this easy to live with 400mm prime.
Drawbacks: I don't think your camera can auto-focus at f8 so with your current setup it would be manual focus only when you add the 1.4x. This lens also doesn't have image stabilization which wasn't a big deal to me since I typically shoot at high shutter speed but it is a limitation if you plan to use it in lower light particularly with the 1.4X. You may need to think tripod or at least monopod.
And as for that Amazon lens, it will create images but probably not images you would want to do anything with beyond posting on social media without your name on them 🙂 There is no free lunch and that old saying is true ten times over with photography gear. With kludges like that you will find yourself struggling to get an even half decent image some of the time with nothing but garbage the rest of the time. Definitely read the negative reviews and decide what quality of output that you need. There are worse things to attach to your camera but you would need to spend some time searching...
Ultimately, no lens is perfect but some are far less imperfect than others. I spent some time this morning trying out a Canon 1.4X III with my EF 800 f5.6 and dialing in the AF calibration because depth of field is extremely shallow with this setup. I will use this combo when needed because sometimes you just can't get close enough but I also accept that the result would be better with just the bare lens if I could get closer.
So no matter what lens you end up using, don't ever let a 1.4X become too much of a convenience crutch. When you are going for best quality, use the extender because you have to in order to get the shot, not because of convenience. My little buddy Woody the almost tame woodpecker was nice enough to model for me this morning because I keep the feeder stocked with his favorite suet cakes. He looks OK with the EF 800 plus 1.4X combo but if he wanted a fashion shot I would close in and use the bare lens 🙂
Rodger
I was a bit skeptical of the 1000mm lens for that price. You get what you pay for. I use autofocus but find myself trying/experimenting with the four manual settings rather than Auto. Wife uses Auto on her ecamera and gets great photos but limited in distance. Ran across a YT video on wildlife photpgraphy. Good excuse to use my ground blind or portable blind. Turkeys are starting to roam and crank up for springtime.
Looked at the 400mm but the price is out of my budget at this time. Was looking at getting the distance from where I shoot from the kitchen to a bird feeder. 300mm is shy for what I need for a good detailed photo compared to the photos I get from the feeder on the deck.
Edit- like Woody's posing for you. Bet he works cheep. 
Here is a sample of my Sigma DG 150-500mm F/5-6.3 APO HSM SLD DG RF OS Lens. This is at 300mm but the lens is this good all the way up to 500mm (or 800mm in your case). I admit, I got a very good copy but even a so-so copy can do very nice pictures. Althoug not a bird check out the hair and eyes.
