Building on the earlier suggestion, sharing your login makes me really nervous, especially if you have your own cards saved on there. Before suggesting the best workaround, does your mom already have her own Walmart account set up with her payment info? If she does, I would suggest making a registry or a custom list on the Walmart site instead of sharing passwords. You can add your groceries to a public registry and send her the link. She can add everything to her own cart from there. It is way safer. TL;DR: Dont share your login. Create a registry with your items and send her the link so she can buy them securely from her own account.

In my experience squeezing every penny out of grocery budgets, Walmart doesn't have a share link. I've always just shared my login and changed the password immediately after she pays.