Póstmaðurinn sagði að rakningarnúmerið væri rétt, en vefsíðan uppfærðist seint.

Those endings are the suffixed definite article in Icelandic, which often works like English the.

So in this sentence:

• póstmaðurinn = the postman
• rakningarnúmerið = the tracking number
• vefsíðan = the website

The exact article ending changes depending on the noun’s gender, number, and case.

Væri is the subjunctive form of vera in the past. Icelandic often uses the subjunctive in indirect speech or reported statements, especially after a verb like sagði.

So:

• Póstmaðurinn sagði að ... væri rétt = The postman said that ... was correct

Using væri shows that this is what was being reported, not necessarily the speaker’s own direct assertion.

A rough comparison:

• er = is
• var = was
• væri = were / was in a reported or less direct sense

After að meaning that, Icelandic uses a subordinate clause, and the word order is more straightforward than in many main clauses.

Here:

• Póstmaðurinn sagði = main clause
• að rakningarnúmerið væri rétt = subordinate clause

Inside the subordinate clause, you get:

• rakningarnúmerið = subject
• væri = verb
• rétt = complement

So the structure is basically that + subject + verb + complement.

Because rétt agrees with rakningarnúmerið, which is neuter singular.

The noun númer is neuter, so when rétt describes it, the adjective appears in the neuter singular form:

• réttur = masculine
• rétt = neuter
• rétt can also be feminine in some contexts, but here it is neuter because of númer

Since this is a predicate adjective after væri, it still has to agree with the noun it refers to.

The base form is rakningarnúmer.

It is a compound:

• rakning = tracking
• númer = number
• -ið = the

So rakningarnúmerið literally means the tracking-number.

Compound nouns are extremely common in Icelandic, so long words like this are normal.

Uppfærðist is the past tense, 3rd person singular form of the middle-voice verb uppfærast, meaning to get updated or to update in an intransitive sense.

So:

• vefsíðan uppfærðist = the website updated / got updated

The ending -st is important. It often marks a middle voice form in Icelandic, which can sound reflexive, passive-like, or just intransitive depending on context.

Compare:

• uppfæra = to update something
• uppfærast = to get updated

Because seint is an adverb, meaning late.

It modifies the verb uppfærðist, not the noun vefsíðan.

So:

• seinn = late, slow (adjective)
• seint = late (adverb)

In other words, the sentence is saying the website updated late, not that the website itself was a late website.

They are all in the nominative, because each one is the subject of its clause:

• Póstmaðurinn = subject of sagði
• rakningarnúmerið = subject of væri
• vefsíðan = subject of uppfærðist

So even though the sentence has multiple clauses, each clause has its own subject in the nominative.

En means but.

It connects two clauses and shows a contrast:

• the postman said the tracking number was correct
• but the website updated late

So en is a coordinating conjunction linking two ideas of equal grammatical status.

A more direct version could be:

Póstmaðurinn sagði: Rakningarnúmerið er rétt.

That would be closer to The postman said: The tracking number is correct.

In the sentence you were given, the statement is turned into indirect speech:

• sagði að ... væri rétt

That shift is one reason you see væri instead of er.

The letter ð is usually like the th in this, not like an English d.

However, in real Icelandic speech it can be quite weak, and its pronunciation depends on position.

In these words:

• sagði
• rakningarnúmerið

you should avoid a strong English d sound. A softer th-like sound is closer, though in fast speech it may sound very light.

Yes: two things stand out.

First, Icelandic often uses the definite article as a suffix rather than a separate word:

• póstmaðurinn
• rakningarnúmerið
• vefsíðan

Second, Icelandic frequently uses the subjunctive in reported speech:

• sagði að ... væri

Both of those are very common patterns, so this sentence is a good example of everyday Icelandic grammar.