Learn Hardcore Turkish: Lütfen peşin ödeme yapınız; bu kampanyada taksit seçeneği yoktur. - Please make a cash upfront payment; there is no installment option in this campaign.

yapınız is the formal/plural imperative. It’s common on signs, instructions, and official notices; polite but also quite firm.
yapın is still polite (addressing siz) but less stiff; good for everyday spoken requests.
yap is the bare imperative (addressing sen); direct and informal.
A softer request is yapar mısınız?, which sounds more like would you please do X?

Why say ödeme yapmak instead of just ödemek?

Both are correct:

ödemek is the simple verb to pay: Lütfen peşin ödeyin.
ödeme yapmak is a light-verb construction literally do/make a payment: Lütfen peşin ödeme yapınız. Signs and formal instructions often use the noun + yapmak pattern; the meaning here is the same.

What’s the difference between peşin and nakit?

peşin = upfront/in advance (not in installments). It’s about timing/terms.
nakit = cash (as a payment method). It’s about the medium (cash vs card). So peşin ödeme can be cash or card, as long as it’s paid in full upfront.

Does peşin ödeme exclude paying by credit card?

No. Peşin ödeme means paying the full amount at once (e.g., card single charge, often called tek çekim), as opposed to installments. It doesn’t require cash specifically.

What exactly does bu kampanyada mean, and why -da (not -de)?

bu kampanyada = in/within this promotion/offer.
The locative suffix is -DA/DE, chosen by vowel harmony. The last vowel of kampanya is a (a back vowel), so you use -da: kampanyada.
You could also say bu kampanya için (for this promotion), which is equally correct but emphasizes purpose rather than location/context.

What does taksit mean, and how do related forms work?

taksit = installment.
taksitle = by/in installments (instrumental): taksitle ödeme.
taksitli = with installments (adjectival): taksitli satış.
Verbs: taksit yapmak (to pay in installments), taksitlendirmek (to put something on installments).

Why is it taksit seçeneği (with -i) and not taksit seçenek?

It’s an indefinite noun compound (belirtisiz isim tamlaması): modifier + head-with-3sg-possessive.

taksit (modifier) + seçenek (head) + 3sg possessive -(s)i → seçeneği (note k → ğ softening).
This does not mark accusative; it marks the possessive relation: option of installments.

Is yoktur different from yok or değildir?

var/yok express existence/non-existence: there is/there isn’t.
yoktur adds the copular -DIr for a formal, assertive tone (typical in notices).
değildir negates an equation/description (X is not Y). You’d use it with a predicate like mevcut değildir (is not available), not directly with yok.

Could I say taksit yoktur instead of taksit seçeneği yoktur?

Yes, taksit yoktur is understandable and concise, but taksit seçeneği yoktur is more explicit (there is no installment option). Other formal variants: taksit uygulanmamaktadır or taksit imkânı yoktur.

Why isn’t it peşin ödemeyi yapınız with accusative -yi?

In general instructions, the direct object is indefinite and stays bare: peşin ödeme yapınız. Using the accusative (ödemeyi) would imply a specific, identifiable payment already known in the discourse.

Can I replace the semicolon with a period or a comma?

Period: Lütfen peşin ödeme yapınız. Bu kampanyada taksit seçeneği yoktur. (Perfectly fine.)
Semicolon: neatly links two closely related statements (as in the original).
Comma alone is not ideal. If you add a conjunction like çünkü, then a comma is fine: Lütfen peşin ödeme yapınız, çünkü bu kampanyada taksit seçeneği yoktur.

Can I change the word order?

Yes, Turkish allows reordering for emphasis:

Bu kampanyada taksit seçeneği yoktur; lütfen peşin ödeme yapınız. (Fronts the context.)
Taksit seçeneği bu kampanyada yoktur. (Emphasizes that the lack applies to this campaign in particular.) Meaning stays the same; emphasis shifts.

How do I pronounce the special letters here?

ö as in German ö or French eu: rounded front vowel (like the vowel in English “bird” but fronted).
ş as English sh.
ı is a high, back, unrounded vowel (no exact English equivalent; a relaxed uh). Examples: peşin (peh-SHIN), ödeme (œ-deh-meh), yapınız (YA-pı-nız), seçeneği (se-che-NE-yi with soft g: eeh glide), yoktur (YOK-toor).

How do I say the affirmative version?

Neutral: Bu kampanyada taksit seçeneği var.
Formal/assertive: Bu kampanyada taksit seçeneği vardır.

What are more formal or more casual alternatives to the whole sentence?

More formal: Peşin ödeme yapmanız rica olunur; bu kampanyada taksit seçeneği yoktur.
Neutral: Lütfen peşin ödeyin; bu kampanyada taksit yok.
Very casual/sign-like: Peşin alıyoruz; taksit yok.
Necessity tone: Peşin ödemeniz gerekiyor; bu kampanyada taksit seçeneği yoktur.