A user interface tag for use in processing a service on a scannable document is provided. A printable surface is on one side of the scannable document and an adhesive surface is on another side of the scannable document. The printable surface further includes a printed data field specified substantially within the printable surface, including machine-readable marks of digital data encoding a service code and a user identification number; and a printed rectilinear border surrounding the printed data field to define a rectilinear iconic representation. A scanned representation of the machine-readable marks is located by identifying the printed rectilinear border using corner candidates oriented in diametric opposition from among connected components identified on the document and the scanned representation of the machine-readable marks are decoded from the rectilinear iconic representation to specify the user identification number and the service code.