Comme l’expéditeur a oublié le code postal, la lettre risque d’arriver en retard.

No. In this sentence, Comme means since or because, not like.

At the beginning of a sentence, comme is often used to give the reason first:

Comme l’expéditeur a oublié le code postal, la lettre risque d’arriver en retard.
= Since/Because the sender forgot the postal code, the letter may arrive late.

This use of comme is most natural when the reason comes first. If you put the cause after the main clause, French usually prefers parce que instead:

• La lettre risque d’arriver en retard parce que l’expéditeur a oublié le code postal.

So here, comme introduces the cause.

L’expéditeur means the sender.

The full article would normally be le because expéditeur is masculine:

• le expéditeur → not possible in normal French
• le becomes l’ before a vowel sound

So:

• le + expéditeur → l’expéditeur

This is called elision. French does this very often:

• l’homme
• l’école
• l’amie

Even though you see l’, the noun is still masculine here.

Expéditeur is usually masculine.

You can tell in this sentence because the article is underlyingly le:

• l’expéditeur = the sender (masculine noun)

The feminine form is usually expéditrice.

So:

• un expéditeur = a male sender / a sender
• une expéditrice = a female sender

In many practical contexts, especially general statements, the masculine form may be used as the default dictionary form.

A oublié is the passé composé, the common French tense for a completed past action.

It is formed with:

• avoir in the present tense
• • past participle

So:

• a = has
• oublié = forgotten

Together:

• a oublié = forgot

Why not the other options?

• oublie = present tense, forgets or is forgetting
• est oublié = is forgotten, which is passive and means something different

So l’expéditeur a oublié le code postal means:

• the sender forgot the postal code

In French, nouns usually need an article more often than in English.

So where English may say:

• forgot postal code

French normally says:

• a oublié le code postal

Here le code postal means the postal code / the zip code relevant to the letter.

French generally prefers:

• oublier quelque chose
• and that something usually comes with an article

Compare:

• J’ai oublié les clés. = I forgot the keys.
• Il a oublié son sac. = He forgot his bag.
• Elle a oublié le numéro. = She forgot the number.

Risquer de + infinitive means to risk doing something, or more naturally in English in many cases, to be likely to or may.

So:

• la lettre risque d’arriver en retard

literally means:

• the letter risks arriving late

But natural English is usually:

• the letter may arrive late
• the letter is likely to arrive late
• the letter risks arriving late (less common, but possible)

This is a very useful structure:

• Il risque de pleuvoir. = It may rain.
• Elle risque d’être en retard. = She may be late.
• Nous risquons de manquer le train. = We may miss the train.

Because the verb risquer is followed by de before another verb in the infinitive.

The pattern is:

• risquer de + infinitive

So:

• risque d’arriver
• risque de tomber
• risque d’oublier

The de becomes d’ before a vowel sound, which is just elision again:

• de arriver → d’arriver

This is normal and required after risquer when another verb follows.

They are related, but not identical.

• arriver en retard = to arrive late
• être en retard = to be late

In this sentence, the idea is specifically about the moment the letter reaches its destination, so arriver en retard is the natural choice.

Examples:

• Le train est en retard. = The train is late.
• Le train arrive en retard. = The train arrives late / is arriving late.

For the letter, French focuses on the delivery event:

• la lettre risque d’arriver en retard
  = the letter may arrive late

The comma separates the reason clause from the main clause.

Structure:

• Comme l’expéditeur a oublié le code postal,
  reason/cause
• la lettre risque d’arriver en retard.
  main result

This is very similar to English:

• Because the sender forgot the postal code, the letter may arrive late.

When the subordinate clause comes first, French usually uses a comma before the main clause.

Yes, but the structure usually changes.

Most natural options are:

• Comme l’expéditeur a oublié le code postal, la lettre risque d’arriver en retard.
• La lettre risque d’arriver en retard parce que l’expéditeur a oublié le code postal.

Starting the sentence with parce que is possible in some contexts, but here comme sounds more natural when the cause comes first.

So a useful rule is:

• comme often introduces a reason at the beginning of a sentence
• parce que often introduces a reason after the main clause

A careful approximate pronunciation is:

Komm lex-pay-dee-tur ah oo-bly-ay luh koad pos-tal, lah let-tr risk dah-ree-vay ahn ruh-tar.

A few points to notice:

• l’expéditeur links smoothly: the l’ connects directly to the next word
• a oublié is pronounced as two vowel sounds in sequence
• risque d’arriver also flows together because of d’
• en retard is a fixed expression, with a nasal sound in en

If you want, you can think of the rhythm in two chunks:

• Comme l’expéditeur a oublié le code postal
• la lettre risque d’arriver en retard

Usually code postal is the standard French term.

For a learner, code postal is the safest and most widely useful choice.

English speakers often think of ZIP code, but that is specifically the American term. In French, even when talking generally, code postal is the normal expression.

So in this sentence:

• le code postal = the postal code / zip code

depending on the variety of English used in the translation.