koverto

Is it related to kovri? Yes, and it does not help you. Both go back to Latin cooperire, to cover over, so the relation is real history. What it is not is Esperanto morphology: you cannot build koverto out of kovri, because there is no suffix -ert- and no rule that would turn "to cover" into "a paper thing you put a letter in". The word arrived complete from German Kuvert, and kovert- is a single indivisible root.

This is the standard shape of a false seam, and it is worth stating in general. A borrowed word often carries visible history from the language it was borrowed from. Reading that history is a pleasure and sometimes a memory aid. Acting on it - splitting the word and expecting the pieces to behave like Esperanto pieces - is the mistake. Compare parkere, where French par coeur is plainly audible and equally inert.

The letter-writing vocabulary, in one place. You write a letero, fold it into a koverto, write the adreso on the front, stick on a poŝtmarko, and take it to the poŝto. The verb for putting it in is enkovertigi, which is built the ordinary way: en + kovert + -ig-.