Hi Sijma, and congratulations on completing the 250 boxes! I can imagine how much you feel relieved, so take a deep breath and enjoy the certainty that you have tamed the beast that this challenge was. I will divide my critique in a couple of sections where I will give you my comments and suggestions.

Linework and confidence

You start your challenge with a good amount of confidence in your construction lines, and throughout the challenge they remain confident and precise, with very little deviation, arching and overshooting whenever present (if at all), and no visible wobbling whatsoever. Overall, I think that you did a very fine job with your construction lines, there's not much to say here. For completeness sake, I will point out a couple of things: first, there are a couple of boxes with either repeated lines on a side of the box that isn't part of the silhouette (such as boxes 11 and 40) or that have more than 2 repeated lines for a single side, likely done to correct an unsatisfactory mark (like in boxes 39, 40, 42 and 47). Remember that in this course, repeated lines are a tool that we should use only in very specific cases and with precise intent: in the context of the challenge, to apply lineweight to the silhouette of the box in order to give it a more solid feeling, and it should only consist of a single repeated line. If a mark doesn't go how we want we shouldn't try to fix it unless strictly necessary, and trying to correct this while working with ink often results in a messy situation that makes the illusion that we are looking at something other than a collection of lines on a piece of paper less believable. Part of the skill set that this course aims to teach or improve is the capacity of accepting and working around our mistakes. Second, your hatching lines in some boxes seem to be done with less care than your construction lines, often resulting wavy and floating inside the face of the box instead of going side to side. In a couple of boxes (as 184 and 185) you even get a bit sloppy and resort to scribbling. Hatching lines are not second rate lines and as such we should treat them the same as any other line, planning and tracing them confidently with the ghosting method.

Construction and perspective

You seem to start the challenge with an already good understanding of how one should intuitively apply 3 point perspective. Your boxes look generally believable as outlines of 3d objects and your line extensions confirm that indeed you achieved a fairly good accuracy for a drawing with perspective established through the naked eye alone. You also experimented with a fair amount of shapes for your boxes, which is good. You also handled pretty well the back corner most of the time. It's still not perfect, but that's to be expected since the back corner is where mistakes in evaluation of perspective tend to pile up if we start drawing our box from the front, be it through rigorous ghosting towards the imagined VP or by placing your endpoints of the segment through visual intuition, the former because of how we construct the box, the latter because we may subconsciously tend to privilege convergence with one side at a time due to the tendency of our brain to simplify things as much as possible. One thing we may do to improve this aspect is to construct the box from the back corner instead, or fixing one direction when we are inspecting one set of sides of the base by placing a provisional dot around where you guess the intersection with the other direction may be, and then check for the other set of sides to improve upon your original guess. The only thing I will criticize here is that I feel you could have experimented a bit more with boxes with shallow degrees of foreshortening, i.e. boxes that, while still having visible perspective deformation, have quite far vanishing points, often lying out of the page, which is a good way to train our sense of perspective intuition since we are not necessarily able to trace back to the vp in that case and we have to think carefully about what looks believable without precise tools.

Conclusion

I think you did an overall very good work with your challenge. To answer two of your questions/notes: regarding the construction method you used, as the goal of the challenge is ultimately understanding how objects look in 3d space and how to replicate that through our geometric intuition, it doesn't hurt that after a certain number of boxes you decided to ditch the complete detailed process, and that's in fact our final goal, sort of like when we try to take off the training wheels at a certain point while learning how to ride a bike. As for your question about why some images where flipped in the uploading process, I'm afraid I don't have a clue.

Anyway, those are my thoughts about your submission. I think you are ready to move on to lesson 2. Good luck and good work!