What's the difference between ～得ない【えない】, as in, say, 届き【とどき】得ない【えない】, and ～eない, as in 届けない【とどけない】? As far as I can tell they both mean "it can't be done," "it's impossible", but is there a difference in connotation or a subtle difference?
届けない doesn't mean "something can't reach" but "to not deliver something" as chocolate says.
Since 届くis an intransitive verb that doesn't stand for one's volitional action, 届く doesn't have the potential form.
As for difference between ～得ない and potential forms, the former stands for possibility under a certain condition while the latter does one's ability or capability. So you can't use potential forms when the subject of a sentence is linguistically inanimate like plants or something that doesn't move by itself, in other words, you can say この木は大きくなり得ない or …大きくならない for "this tree can grow big" but not この木は大きくなれない (in average people's sense).