Abstract: A system for making secure transactions by mail-order purchasing, in particular on the Internet, with delivery of a unique and non-reusable code for each completed transaction. The system involves a third party (20, 50) between the purchaser (10) and the seller (30, 60). The third party has a table (80) likewise stored in an electronic fill device (70) of the purchaser (10). The third party validates the purchase when the code, issued from the electronic fill device (70) and transmitted by the purchaser, is identical to a code present in the table located at the third party's. The code advantageously comprises the value of an incremental counter associated with a certification number randomly determined when the electronic fill device (70) is initialized.