Structuralism and post-structuralism is, roughly speaking, a distinction without a difference, so we can look at them together.
In your question, you state:
From a naive point of view, these ideas say that (1) we can interpret the text (2) as we want (3) since multiple interpretations are possible. (4) But however, to me, the possibility of multiple interpretations does not mean that all interpretations are equally valid. To me, it seems like (5) we can always fall back on the possible most likely interpretations or (6) most likely intention of the author, (7) given the available evidence and context.
Moving piece by piece,
(1) S/PS does agree that we interpret texts. Here the point is that every time we encounter a text we are engaging in interpretation. i.e., we have no unmediated access to the text.
(2) S/PS rejects the idea that these interpretations are "as we want." Instead, the interpretations arise from our understanding of language which is part of what mediates our encounter with the text.
(3) S/PS agrees to some extent that multiple interpretations are possible, but here we need to be careful, because S/PS does not think these are interpretations approximating the "true meaning" of a passage.
(4) For S/PS, the multiple interpretations occur because of the structures we have in us -- or rather the structures construct the interpretations.
(5) For S/PS, the so-called "most-likely interpretation" is the one that the structures of our culture have made seem most likely -- i.e., the one that fits best with our preconceptions.
(6) For S/PS, the author is unimportant to the meaning of the text, because what is produced is a consequence of the structures moving through time. We have no access to the author's original intent, and there is no such thing on PS.
(7) For S/PS, the "available evidence and context" is just a mirage for the things our culture encourages us to interpret as so.
Maybe, to put things in context in a hyper-simplified way--
S/PS is a movement that builds on a key Kantian theme: the idea that we have no access to the things in themselves but only as they are mediated through our perceptive and cognitive faculties. Passing through a bit of Hegel, it turns out the ways our faculties work is not static (as Kant seemed to imagine) but rather have changed over time.
Hegel also has the interesting claim that what we are really discovering in all of this thinking is that we are thinking, so what we are discovering is our thoughts. Hegel is still a realist about things mind you.
The S/PS lop off that realism. All we have left them are structures of thinking and comprehension that change over time. There's no meaning inherent to be discovered in books themselves on this view; there's just the task of interpreting things as they evolve.
Here's a few sources that look okay: