It happened at the store's location near the Galleria.
Kristina said John, the shopper, accidentally left his change behind after she rang him up, which totaled out to $194.
She called John to let him know he left the money behind.
John and his wife, Barbara, thought it was a scam at first, but when they went back, they met with Kristina, who had his cash in her hand.
"He didn't realize that he had forgotten the money," Barbara said.
Kristina posted a picture of John, which was featured on the Costco's official Instagram page and has already garnered nearly 10,000 likes.
John and his wife said the entire staff at the store are wonderful and they are grateful for their honesty.
"In these stressful times, we have been so very impressed with Kristina and all employees at this Costco. From the excellent staff at the pharmacy to the greeters, cashiers and stockers." Barbara said. "Most of the pharmacy employees and greeters know us by name. Now, we are delighted to add Kristina to our friend list."