We have always ordered from their website.
It is easy and requires nothing special. WhatsApp wouldn't work at all.
https://www.walmart.com.mx/inicio After you create your account, use the dropdown menu on the far left next to "Walmart" and click "Mismo Dia", sign-in, and thenyou are ready to fill your cart. It's all pretty straight forward from there. I pay with PayPal, but you can pay cash to your driver if you like or by CC.