To make a payment on your Walmart Mastercard, you can use the Capital One mobile app, or also by phone by dialing (800) 227-4825, via mail, or in person at any Walmart store.
Regarding payment processing, every card payment is usually posted within 2 business days. So, if you submit your payment by 8 p.m. ET, Monday through Saturday, "it will be posted by midnight on the same day it's received. Otherwise, it will post the following day, by midnight."