There is a cash limit that you can pay at OXXO, (in the past, it was 10,000 pesos), however, you can look into Mercado Pago an extension of Mercado Libre which does allow for higher amounts. At the checkout point in Mercado Libre if you choose that you want to pay casn a list of options pops up. These usually include a number of banks and stores what will allow a direct payment. I have used both OXXO and Citibanamex.