Wie vergoedt kosten als niemand het pakket aflevert?

Because wie is the subject here, and Dutch uses the third-person singular verb form with it.

The infinitive is vergoeden.
Its stem is vergoed.
For third-person singular present tense, Dutch adds -t:

• ik vergoed
• jij vergoedt
• hij/zij/het vergoedt
• wie vergoedt ...?

The spelling vergoedt can look strange, but it is correct: stem vergoed + t.

Because wie is the subject of the sentence.

In Dutch, if the question word is the subject, the word order stays similar to a normal statement:

• Wie vergoedt kosten ...?

Compare that with a question where the question word is not the subject:

• Wat vergoedt de verkoper?

There, de verkoper is the subject, so it comes after the verb.

This is similar to English:

• Who pays?
  but
• What does the seller pay?

A learner is very likely to notice this, because de kosten is often the more natural choice in everyday Dutch.

• de kosten vergoeden = reimburse the costs / the expenses
• kosten vergoeden = reimburse costs, more general or somewhat more formal/compact

So the given sentence is understandable, but many speakers would naturally say:

• Wie vergoedt de kosten als niemand het pakket aflevert?

If specific costs are meant, de kosten is usually clearer and more idiomatic.

Both can involve money, but they are not the same.

• betalen = to pay
• vergoeden = to reimburse, compensate, or cover costs

So vergoeden is used when someone pays back expenses or takes responsibility for them.

For example:

• De klant betaalt de rekening. = The customer pays the bill.
• Het bedrijf vergoedt de kosten. = The company reimburses/covers the costs.

In this sentence, vergoeden suggests covering the costs rather than simply making a payment.

Here als means if.

It introduces a condition:

• als niemand het pakket aflevert = if nobody delivers the package

Dutch often uses als for ordinary conditional sentences. Other possibilities are:

• indien = if, but more formal
• wanneer = when, and sometimes also if, depending on context

In everyday Dutch, als is the most normal choice here.

Because afleveren is a separable verb.

Its parts are:

• af
  • leveren

In a main clause, the prefix usually separates:

• Niemand levert het pakket af.

In a subordinate clause, the verb goes to the end and the parts stay together:

• ... als niemand het pakket aflevert.

This is one of the most important patterns in Dutch verb word order.

Because als introduces a subordinate clause, and in Dutch subordinate clauses the finite verb normally goes to the end.

So:

• main clause: Niemand levert het pakket af.
• subordinate clause: als niemand het pakket aflevert

This final-verb pattern is very common after words like:

• als = if
• omdat = because
• dat = that
• wanneer = when

Because pakket is a het-word in Dutch.

So you get:

• het pakket = the package
• een pakket = a package

This is something you mostly have to learn noun by noun. There is not always a reliable rule from meaning alone.

Also note:

• singular definite: het pakket
• plural definite: de pakketten

Because niemand already means nobody / no one.

So:

• niemand = no one
• niemand levert het pakket af = nobody delivers the package

You do not add another negative like geen with it in this kind of sentence.

This is similar to English standard usage:

• Nobody delivered it not
• Nobody didn’t deliver it

It is grammatical, but in real delivery/customer-service Dutch, many speakers might prefer a passive version:

• Wie vergoedt de kosten als het pakket niet wordt afgeleverd?

Why? Because in practice, the focus is often on the package not being delivered, not on nobody delivering it.

So the original sentence is fine, but it can sound a little more natural in context if you use:

• de kosten
• and possibly the passive het pakket niet wordt afgeleverd