Adding someone else's credit card to my Apple Pay with their permission

I am my mother's caretaker. She will often give me her credit card to go pick up things for her like food and medicine. I have an iPhone 8+. My question is can I add my mother's credit card information to my Apple Pay with my mother's permission and just use this digital card stored in the wallet app when my mother tells me to go buy something for her and charge it to her mastercard. Am I able to do this w/o requiring my mother to make me an authorized user on her credit card? I have all my mother's banking and credit card information ( I set all this up for her because my mother is unable to do) and she tells me when to pay most of her bills electronically using either my smartphone or desktop PC. Thanks--