1. A user interface tag for use in processing a service on a scannable document, comprising: a printable surface on one side of a scannable document; an adhesive surface on an other side of the scannable document, wherein the printable surface further comprises: a printed data field specified substantially within the printable surface and comprising 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, wherein 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; a plurality of extreme points extending furthest in a selected direction in each of the connected components, wherein the corner candidates meet a diagonal length criteria applied to at least one pair of the extreme points; and a plurality of coordinate pairs (x, y) to represent each of the extreme points, which are each determined as: 
 N=( x N ,y N )ε C|y N ≦y ∀( x,y )ε C, 
 E=( x E ,y E )ε C|x E ≧x ∀( x,y )ε C, 
 S=( x S ,y S )ε C|y S ≧y ∀( x,y )ε C, 
 W=( x W ,y W )ε C|x W ≦x ∀( x,y )ε C, 
 NE=( x NE ,y NE )ε C |( x NE −y NE )≧( x−y )∀( x,y )ε C, 
 SE=( x SE ,y SE )ε C |( x SE +y sE )≧( x+y )∀( x,y )ε C, 
 SW=( x SW ,y SW )ε C |( x SW −y SW )≦( x−y )∀( x,y )ε C, and 
 NW=( x NW ,y NW )ε C |( x NW +y NW )≦( x+y )∀( x,y )ε C, where N is a northernmost point defined by coordinates x N and y N , E is a easternmost point defined by coordinates X E and y E , S is a southernmost point defined by coordinates x S and y S , W is a westernmost point defined by coordinates x W and y W , NE is a north-easternmost point defined by coordinates x NE and y NE , SE is a south-easternmost point defined by coordinates x SE and y SE , SW is a south-westernmost point defined by coordinates x SW and y SW , NW is a north-westernmost point defined by coordinates x NW and y NW , and C is set of pixels representing the connected components.