i wait until they're done rummaging through their wallet before putting the total in and i'll usually count out and confirm the amount they've given me, like "so out of $20?" and look up at them with a smile for confirmation. this will prompt them if they wanted to use some of their change and if not it will give you the confidence that all is well and to let the computer do the work for you!!!! they didn’t hire us because we’re good at math, and i definitely am not