I rarely carry cash anymore, if I do, it's almost always 20s. It would be great if the Marriott App would let us tip the housekeeping staff via the app. Starbucks has this ability with their app. The option for those of us who no longer carry cash is skip the tip or go find an ATM and have the 20s broken into smaller by someone else. The integration of tipping into the app would solve that problem and ensure better care of the housekeeping staff.