But intentions are not binding contractual conditions. Nor need they lead up to binding contractual conditions. Moreover a letter expressing intent, in its form, does not adequately suggest what the appropriate response should be. Whereas a Request for Quotation / RFQ quite literally is a Request..
Finally, there is no such thing as an oral offer! An oral offer is really just a test by the buyer to see what the seller will or will not accept. Make the buyer put the terms of the offer in writing as an intermediate step toward securing a legitimate offer.