Traceer het pakket voordat ze terugbetalen.

Because Traceer is an imperative: a command or instruction.

In Dutch, imperatives usually do not include the subject:

• Traceer het pakket. = Track the package.
• Wacht hier. = Wait here.
• Lees dit. = Read this.

So the subject is understood as you.

It is the imperative form of the verb traceren.

For many Dutch verbs, the imperative is based on the stem:

• traceren → traceer
• wachten → wacht
• betalen → betaal

So Traceer! means Track!

Because pakket is a het-word in Dutch.

Dutch nouns take either de or het:

• de tafel
• de auto
• het huis
• het pakket

Unfortunately, this is something you often just have to learn with the noun. So the correct form is:

• het pakket

Because voordat is a conjunction meaning before when it introduces a full clause.

Here it introduces:

• voordat ze terugbetalen

So it is written as one word.

Compare:

• voordat ze terugbetalen = before they refund/pay back
• voor dat pakket = for that package

In voor dat pakket, voor and dat are separate because dat is not part of a conjunction there.

Ze is the normal unstressed subject pronoun for they.

Dutch often uses:

• ze = unstressed they
• zij = stressed they

So in a neutral sentence, ze is very common:

• Ze komen morgen. = They’re coming tomorrow.

You would use zij if you want emphasis or contrast:

• Zij betalen terug, niet wij. = They are refunding, not us.

Not in this sentence.

Ze can mean either:

• she
• they

But the verb tells you which one it is.

Here the verb is terugbetalen, which is plural, so ze must mean they.

Compare:

• voordat ze terugbetalen = before they refund/pay back
• voordat ze terugbetaalt = before she refunds/pays back

Because voordat introduces a subordinate clause, and in Dutch subordinate clauses the verb typically goes to the end.

Main clause:

• Ze betalen terug.

Subordinate clause:

• voordat ze terugbetalen

This is a very common Dutch pattern:

• omdat hij werkt
• als ik tijd heb
• terwijl ze slapen
• voordat ze terugbetalen

So the end position of the verb is completely normal here.

Because terugbetalen is a separable verb.

Its parts are:

• terug
  • betalen

In a main clause, separable verbs are often split:

• Ze betalen morgen terug.

In a subordinate clause, they usually stay together:

• ... voordat ze terugbetalen

So:

• main clause: ze betalen terug
• subordinate clause: voordat ze terugbetalen

That is why you see one word here.

Yes, very possibly.

Terugbetalen often has an object, such as:

• het geld = the money
• je = you
• het aankoopbedrag = the purchase amount

For example:

• Ze betalen het geld terug.
• Ze betalen je terug.

In your sentence, the object is not stated, but it can be understood from the context. Dutch often leaves things out when they are already clear.