I've been in BA for a month (on a tourist visa) and have been paying for everything in cash (after wiring pesos to myself with Western Union). I want to book flights online within the country but I would be forced to use my Canadian credit card and paying double (since I'd be forced to use the official exchange rate). I tried to sign up for Mercado Pago but they require a DNI which I don't have.
Is there a way to pay online with pesos for non-residents?
For flights see if you can pay in a branch here in the city, as for other purchases, i.e. physical goods, some companies accept payment via PagoFacil/RapiPago/ProvinciaNET, in which case you go to any location and pay in cash with the payment number.
Others want transferencia, which I've only ever done from my/my husband's account online, so I'm not sure if this is possible still, but at one time you could do a payment at the recipient's bank's ATMs where you enter their account number and deposit the cash to their account via the ATM, though given KYC regulations I doubt this is possible still.
As for DNI, previously when I didn't have one I just put my passport number with leading 0s.