Una volta controllata la paga base, l'azienda controfirma.

Here una volta does not mean one time in the literal counting sense.

In this sentence, una volta + past participle / clause means once, after, or when ... has been ....

So:

• Una volta controllata la paga base = Once the base pay has been checked
• very close in meaning to Dopo aver controllato la paga base in some contexts, though the structure is different

This is a very common Italian way to introduce something that happens first, before the main action.

Because it agrees with la paga base, which is feminine singular.

• la paga = feminine singular
• therefore the past participle becomes controllata

This is happening because the phrase is a shortened passive-type construction:

• Una volta che la paga base è stata controllata, l'azienda controfirma.

That longer version makes the agreement easier to see:

• la paga base → feminine singular
• è stata controllata → feminine singular participle

So in the shorter version, controllata still matches la paga base.

Yes. Italian often shortens this kind of clause by leaving out words that are understood.

The full version would be:

• Una volta che la paga base è stata controllata, l'azienda controfirma.

The sentence you have is a more compact version:

• Una volta controllata la paga base, l'azienda controfirma.

So this is basically a reduced clause. English does something similar:

• Once the base pay has been checked, the company countersigns.

The person doing the checking is not stated. The focus is on the action being completed before the next step.

The sentence does not say.

That is one important effect of this structure: it leaves the agent unspecified.

So the meaning is roughly:

• Once the base pay has been checked, the company countersigns.

Not:

• Once the company has checked the base pay...

The checker could be a manager, HR, payroll, another department, or just whoever is responsible. Italian is often happy to leave that unstated if it is not important.

Because controllata la paga base is part of a fixed-style reduced clause meaning once the base pay has been checked.

In this structure, the participle often comes before the noun:

• Terminato il lavoro, siamo usciti.
• Ricevuta la conferma, procederemo.
• Controllata la paga base, l'azienda controfirma.

If you say la paga base controllata, that sounds more like a noun phrase, meaning something like the checked base pay, not a temporal clause.

So:

• controllata la paga base = once the base pay has been checked
• la paga base controllata = more like the base pay that has been checked

The first is what fits this sentence.

Controfirma means countersigns.

That usually means one signature is added after another person has already signed or approved something. So it is not just a normal signature; it is a second or confirming signature.

Compare:

• firma = signs
• controfirma = countersigns

In business, legal, and administrative language, controfirmare is common when a document needs an additional signature from another party, often to validate or confirm it.

So the sentence suggests a process like this:

1. the base pay is checked
2. the company countersigns

Italian often uses the present tense to describe:

• routine procedures
• instructions
• standard sequences
• general facts

So l'azienda controfirma can mean:

• the company countersigns
• the company then countersigns
• the company will countersign

The exact English translation depends on context.

If this sentence appears in a document explaining a process, the present tense is completely natural in Italian.

Paga base means base pay or basic pay.

It refers to the standard amount paid before extras such as:

• bonuses
• overtime
• allowances
• commissions

It is similar to stipendio base, but the nuance can depend on context:

• paga often relates to pay/wages more generally
• stipendio is often used for salary, especially regular salaried pay

In payroll or employment contexts, both may appear, but paga base is perfectly natural.

The comma is very normal here because the sentence begins with an introductory clause:

• Una volta controllata la paga base, l'azienda controfirma.

The comma separates:

1. the preliminary action
2. the main action

Without the comma, the sentence would be harder to read. In standard written Italian, the comma is the expected punctuation here.

Yes. That is a fuller, more explicit version, and it is grammatically correct.

Compare the two:

• Una volta controllata la paga base, l'azienda controfirma.
• Una volta che è stata controllata la paga base, l'azienda controfirma.

The shorter version is more compact and often sounds more formal or procedural. The longer version is more explicit and may be easier for learners to understand.

Both mean essentially the same thing.

Yes, it sounds fairly formal or administrative.

Reasons:

• controfirmare is a formal/business verb
• the reduced clause una volta controllata... is concise and typical of procedural or official language
• the overall sentence sounds like part of a workflow, policy, or document-handling process

In everyday conversation, people might choose a more explicit wording, but in professional contexts this sentence is very natural.