imho

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Note added at 9 мин (2012-10-28 11:31:32 GMT)
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См., напр.:

http://www.exporthelp.co.za/documentation/proforma.html

http://www.google.ru/search?q=offer invoice&hl=ru&newwindow=...

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Note added at 13 мин (2012-10-28 11:34:47 GMT)
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http://www.docstoc.com/docs/33309119/SAMPLE-PROFORMA-INVOICE...

http://www.contractiva.com/images/screenshots/invoice.jpg

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Note added at 19 мин (2012-10-28 11:41:07 GMT)
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In trade transactions, a pro forma invoice is a document that states a commitment from the seller to sell goods to the buyer at specified prices and terms. It is used to declare the value of the trade. It is not a true invoice, because it is not used to record accounts receivable for the seller and accounts payable for the buyer. Simply, a 'Proforma Invoice' is a Confirmed Purchase Order where buyer and Supplier agree on the Product Detail and cost to be shipped to buyer. A sales quote is prepared in the form of a pro forma invoice which is different from a commercial invoice. It is used to create a sale and is sent in advance of the commercial invoice. The content of a pro forma invoice is almost identical to a commercial invoice and is usually considered a binding agreement although the price may change in advance of the final sale.

A pro forma invoice can also be used for shipments containing items that are not being bought or sold, such as gifts, samples and personal belongings, whereas a Commercial Invoice is used when the commodities shipped are being bought or sold. [3]

Banks usually prefer a pro forma invoice to a quotation for establishment of a letter of credit or for advance payment by the importer through his bank.

In some countries, customs may accept a pro forma invoice (generated by the importer and not the exporter) if the required commercial invoice is not available at the time when filing entry documents at the port of entry to get goods released from customs. The U.S. Customs and Border Protection, for example, uses pro forma invoices to assess duty and examine goods, but the importer on record is required to post a bond and produce a commercial invoice within 120 days from the date of entry. If the required commercial invoice is needed for statistical purposes, the importer has to produce the commercial invoice within 50 days from the date Customs releases the goods to the importer.
http://en.wikipedia.org/wiki/Pro_forma

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Note added at 59 мин (2012-10-28 12:21:03 GMT)
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To: ASKER.

Then, probably, in your context 'offer invoice' would do?

Ref. to:
http://www.ubercart.org/forum/support/2730/offer_invoice_ord...

OR

http://askvu.vu.edu.au/app/answers/detail/a_id/133/~/i-have-...

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Note added at 1 час (2012-10-28 12:39:13 GMT)
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What does "счет-оферта" mean in Russian?

"Согласно ГК РФ моментом заключения договора можно считать не только время, когда на договоре поставлена подпись. Договор можно считать заключенным в момент оплаты товара (акцепт), например. Именно поэтому мы не работаем с договорами в привычном смысле этого слова, а используем упрощенную схему, которая не требует пересылки и подписания печатных договоров. Со своими клиентами мы работаем по схеме "счет-оферта".

Счет-оферта в нашем случае - это счет, в котором помимо наименования товара, его количества и цены, прописаны также основные условия договора поставки: условия оплаты, сроки оплаты, а также сроки и условия отгрузки товара. Высылая такой счет нашему клиенту мы делаем наше коммерческое предложение (оферту), а оплата данного счета означает, что клиент согласен с этими условиями (акцепт). Таким образом между нами возникает договор, соглано которому мы обязаны в определенные счетом-офертой сроки отправить клиенту оплаченный им товар."
http://chydogubka.umi.ru/optovikam/chto_takoe_schet-oferta_i...

Hope this will help you to find an exact equivalent in English.

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Note added at 8 час (2012-10-28 20:02:10 GMT)
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Just as an 'idea' (though NOT supported by any dictionaries / glossaries that I've had time to check) but 'reflecting' the 'gist' of this notion in Russian:

Offer Acceptance Invoice.

Why not? We already have on the Internet 'Offer Acceptance Payments' - http://www.qut.edu.au/international/applying/step-8-accept-y...