Mi resendis ĝin sen ke la retbutiko konfirmis la proceduron.

Resendis breaks down like this:

• re- = again, back
• send- = send
• -is = past tense

So resendi means to send again or to send back, depending on context.

In this sentence, it clearly means sent back. That is useful for English speakers because written English resent usually means felt bitterness, while Esperanto resendis is purely the verb from sendi with the prefix re-.

Ĝin is the accusative form of ĝi.

• ĝi = it
• ĝin = it, as a direct object

Here it is the thing being sent back, so it must take -n because it is the direct object of resendis.

So:

• Mi resendis ĝin = I sent it back

The sentence assumes that the object has already been mentioned earlier, so ĝin refers back to that known thing.

Because the second action has its own subject.

• Main clause subject: mi
• Subordinate clause subject: la retbutiko

When Esperanto wants to say without followed by a whole clause, especially when that clause has a different subject, sen ke is a very common structure.

So:

• sen ke la retbutiko ... = without the online store ...

If you changed it to an infinitive, the meaning would change. For example:

• Mi resendis ĝin sen konfirmi la proceduron
  = I sent it back without confirming the procedure

That would mean I did not confirm it, not the store.

Also, in this expression, ke is not best understood as a simple word-for-word that. The whole combination sen ke functions together as without ...-ing / without the fact that ...

Because sen already gives the negative idea.

Sen ke la retbutiko konfirmis la proceduron means roughly:

• without the online store confirming the procedure

You do not normally add ne there, because that would change the logic.

Compare:

• sen ke li venis = without him coming
• sen ke li ne venis = without him not coming

The second version creates a kind of double negative idea and can suggest the opposite meaning. So in your sentence, no ne is needed.

This is a very good question, because many learners notice that konfirmis feels a bit unexpected after sen ke.

In practice:

• konfirmis presents the clause as a past fact from the speaker’s point of view
• konfirmu can sound more natural when the action did not actually happen and is being presented as something expected, possible, or unrealized

So many speakers would prefer:

• Mi resendis ĝin sen ke la retbutiko konfirmu la proceduron.

That version often sounds more idiomatic for I sent it back without the online store confirming the procedure.

So the original sentence is understandable, but konfirmu is very likely the version a teacher or careful speaker would recommend.

Retbutiko is a compound word:

• ret- = internet, web
• butiko = shop, store

So retbutiko means online store or internet shop.

The article la is used because the speaker has a particular store in mind, not just any online store. In other words, it means the online store, one already known from the context.

Esperanto uses compounds like this very freely, so retbutiko is completely normal.

Because proceduron is the direct object of konfirmis / konfirmu.

• konfirmi ion = to confirm something
• that something takes -n

So:

• la proceduro = the procedure
• la proceduron = the procedure, as a direct object

The la shows that it is a specific procedure, probably the return procedure already known in the situation.

Esperanto usually does not need a separate past perfect form the way English often does.

English might say:

• I sent it back without the online store having confirmed the procedure
• or ... without the online store had confirmed ... is ungrammatical in English, so usually having confirmed or after it had confirmed

In Esperanto, the ordinary past tense -is often covers both confirmed and had confirmed, and the time relationship is understood from context.

So if someone keeps konfirmis, the idea of earlier past time is usually clear enough from the sentence structure.

If you want to make earlier timing clearer, you can add words like:

• jam = already
• antaŭe = earlier, beforehand

But often that is unnecessary.

One important point: if you choose konfirmu instead, that is a question of mood, not of past-perfect tense.