If you do find that knowing distance from the tote would be useful (like if you wanted to make sure the robot was square to the tote), I'd suggest trying out IR ranging sensors.
TherTheres a good overview of the Sharp sensors here:
Http://www.acroname.com/articles/shape.HTML
We've found uses for them on all our robots since 2012, and will likely be using them again this year.
You could buy a few with the digikey PDV that came in the KOP. They are also sold at many online retailers.
Http://Pololu.com
Http://SparkFun.com
Http://Adafruit.com
Http://Octopart.com
Make sure you get 3pin JST cables with them, otherwise you'll be soldering wires to these...
Some things to note. Some of the plastic housings on these sensors are conductive, so make sure they are isolated from the chassis.
Also. The voltage drops as objects get closer than the minimum specified range, this could lead to problems with your code reporting incorrect distances. Either mount them far enough in to your chassis so you can't get invalid values, or handle the condition gracefully in your code.