Uniform squishy infill
As the rubber stamp needs to be soft in one axis for the whole area, you could use an infill that causes the same softness in all directions, but is sliced as such that the stamp experiences the same softness. Alternatively you can use the specific infill types for flexibility, but beware of the orientation:


Concentric
Cross
Cross 3D 


First, to get the same softness in each direction you need to use an infill pattern that has similar/uniform properties (isotropic) in all dimensions. 

It is suggested to look into the infill type called "gyroid" (see question What are the advantages of gyroid infill?). 

This type of infill is described as:


  Gyroid infill is one of the strongest infill types for a given weight, has isotropic properties, and prints relatively fast with reduced material use and a fully connected part interior.  


Second, since the stamp has relief, slicing the part may cause different infill height. You could look into Different infill in the same part to e.g. get a solid infill for under the relief to get a uniform infill for the "cushion".