Facciamo un'offerta perché il prezzo è trattabile.

It can be either one, because Italian uses the same form for:

• we make / we do (present indicative)
• let's make / let's do (first-person plural imperative)

So the exact meaning depends on context and tone. In a sentence like this, it often sounds like a suggestion: let's make an offer.

Italian usually omits subject pronouns because the verb ending already shows the subject.
The ending -iamo tells you the subject is we.

So:

• Facciamo un'offerta = We make an offer or Let's make an offer
• Noi facciamo un'offerta is possible, but noi adds emphasis or contrast

Because offerta begins with a vowel, una is commonly shortened to un' before it:

• una casa
• un'offerta

The apostrophe shows that the final -a of una has dropped.

This is only for the feminine singular article una. Compare:

• un'offerta = feminine
• un amico = masculine, with no apostrophe

Because fare un'offerta is the normal idiomatic expression, just like English make an offer.
Italian often uses fare + noun in expressions where English does something similar.

So this is not unusual at all; it is the standard way to say it.

The accent is part of the standard spelling: perché.

It is written with an accent because the stress falls on the last syllable: per-CHÉ. In Italian, words stressed on the final syllable are often written with an accent mark.

In this sentence, perché means because.

Because è and e are two different words:

• è = is
• e = and

The accent is very important in writing.
So il prezzo è trattabile means the price is negotiable, not the price and negotiable.

Italian uses articles more often than English does. Here il prezzo refers to the specific price being discussed, so the definite article is natural.

Using prezzo without an article would be ungrammatical here.
Using un prezzo would change the meaning to something more like a price, which is not the idea in this sentence.

Not all masculine adjectives end in -o.
Some adjectives, including many ending in -abile or -ibile, have:

• one singular form in -e for both masculine and feminine
• one plural form in -i for both masculine and feminine

So:

• prezzo trattabile = masculine singular
• offerta trattabile = feminine singular
• prezzi trattabili = masculine plural
• offerte trattabili = feminine plural

Here trattabile means negotiable, open to bargaining, or not completely fixed.

With prezzo, it means the seller may be willing to change or lower the price.
It comes from trattare, which can mean to negotiate in this kind of context.

Yes, it can. For example:

Perché il prezzo è trattabile, facciamo un'offerta.

That is grammatically possible. However, in everyday Italian, the version with the main clause first often sounds more natural:

Facciamo un'offerta perché il prezzo è trattabile.

Usually, no.
In a sentence like this, Italian normally does not use a comma before perché when the reason clause is closely connected to the main clause.

So the standard version is:

Facciamo un'offerta perché il prezzo è trattabile.

A comma might appear for special emphasis or rhythm, but it is not the default choice.