I'm pretty sure every single German supermarket chain measures that, it's just a requirement to operate at those margins. The Edeka cooperative published in 2012 that their all-in cash handling cost is 0.14%. The 2013 Steinbeis Cost of Cash study[0] estimated a much higher number of 2.7% for merchants. The truth is probably somewhere in between.

Such studies have also been used to arrive at the EU interchange cap.

[0]: http://www.steinbeis-research.de/images/pdf-documents/CFP_Co...