What's the cheapest way to send my sister money to Walmart

Money gram is a good one

Try one of them phone apps or She buys one of them cards u can reload and u add

You can use MoneyGram which is almost instantly or you can go somewhere else and use Western Union Western Union I definitely recommend over Walmart or you can get a Walmart prepaid card or she can and give you the info and you can deposit money in her account on the prepaid Walmart Visa