Vraag bij de bezorger na hoe je dit terugstuurt.

Navragen is a separable verb meaning to ask/check for information, to inquire, or to verify by asking.

So:

• navragen = the dictionary form
• Vraag ... na = the imperative form with the separable particle split off

In this sentence, Vraag bij de bezorger na ... means something like:

• Check with the delivery person
• Ask the delivery person for clarification
• Find out from the delivery person

It is a bit more specific than plain vragen, because it suggests getting information confirmed or clarified.

Because navragen is a separable verb.

In Dutch main clauses and commands, the verb stem comes earlier and the separable particle goes later:

• Ik vraag het na.
• Vraag het na.

But in infinitives and subordinate clauses, the parts stay together:

• Ik wil het navragen.
• ... hoe je dit navraagt (if that were the intended verb there)

So in your sentence:

• Vraag = imperative verb
• na = the separated particle belonging to navragen

Here bij means with, from, or at the place/person where you can get the information.

So bij de bezorger navragen means:

• check with the delivery person
• ask the delivery person
• inquire with the delivery person

Using aan would focus more literally on directing a question to someone:

• Vraag het aan de bezorger. = Ask the delivery person.

Both ideas are possible in Dutch, but bij ... navragen is a very common pattern when the meaning is check with someone / verify with someone.

Bezorger usually means delivery person, courier, or person who delivers something.

Depending on context, it could be:

• a parcel delivery person
• a food delivery person
• someone responsible for bringing the item

In this sentence, it most likely means the person or service responsible for the delivery, and the idea is that they can tell you how to return the item.

Vraag is the imperative form, used for giving an instruction or command.

The verb is vragen:

• ik vraag
• jij vraagt
• imperative: vraag!

So Vraag bij de bezorger na ... means Ask/check with the delivery person ...

It is directed at you:

• Vraag ... = Ask ...

If it were a normal statement, it would be something like:

• Je vraagt bij de bezorger na hoe je dit terugstuurt.

Because this is an embedded question (an indirect question), not a direct question.

Compare:

• Direct question: Hoe stuur je dit terug?
• Embedded question: ... hoe je dit terugstuurt.

In embedded questions, Dutch word order changes:

• the subject je comes before the verb
• the finite verb goes toward the end

So:

• hoe stuur je dit terug? = direct question
• hoe je dit terugstuurt = indirect question

That is why the sentence says:

• Vraag bij de bezorger na hoe je dit terugstuurt.

not

• Vraag bij de bezorger na hoe stuur je dit terug.

Because terugsturen is also a separable verb:

• terugsturen = to send back / return

In a main clause, it splits:

• Je stuurt dit terug.

In a subordinate clause, it stays together:

• ... hoe je dit terugstuurt.

So:

• sturen ... terug in a main clause
• terugsturen / terugstuurt in an infinitive or subordinate clause

That is exactly what you see here.

Yes, je here means you.

It can be understood in two ways:

1. the person being spoken to

   • how you return this

2. generic you

   • how one returns this
   • how you go about returning this

In everyday Dutch, je is very often used where English uses ordinary you, even when the meaning is fairly general.

Dit means this and refers to something close in context: usually a package, product, or item that has just been mentioned or is physically present.

So:

• dit terugstuurt = return this / send this back

If the item were farther away or less immediate in the conversation, Dutch might use dat instead.

Yes, it is natural Dutch.

It sounds like an instruction telling someone to check with the delivery person about the return procedure.

Some similar alternatives are:

• Vraag de bezorger hoe je dit terugstuurt.
  Slightly more direct: Ask the delivery person how to return this.

• Vraag bij de bezorger hoe je dit terugstuurt.
  Also possible, but navragen makes the idea of checking/inquiring clearer.

• Informeer bij de bezorger hoe je dit kunt terugsturen.
  A bit more formal: Inquire with the delivery person how you can return this.

So the original sentence is perfectly good, especially if the speaker wants to stress checking for the correct procedure.

Usually, the most natural order is:

• Vraag bij de bezorger na ...

Dutch separable particles often come late in the clause, and other elements can appear between the verb and the particle.

So this pattern is normal:

• Ik vraag het morgen bij hen na.
• Vraag het bij de bezorger na.

Putting na immediately after vraag is not impossible in every context, but here it sounds less natural than Vraag bij de bezorger na ....

So for a learner, the safest version is:

• Vraag bij de bezorger na hoe je dit terugstuurt.