Being patient with the older customer counting out exact change is a small sermon in a checkout line.

Patience is mostly the decision to let somebody be slow in front of you. The older customer counting out coins knows exactly how long she is taking and is probably apologising for it, and your face in that moment is the whole event for her. There is nothing to achieve here, no efficiency to recover that matters. She gets to pay for her shopping without being made to feel like a nuisance.

On your next shift, wait without sighing or looking past them. Say something ordinary while they count. It costs ninety seconds and it is the best thing you will do that day.