I've been working for foreign clients from Argentina for years, and I agree the Monotributo is a good system. Easy to sort out, no accountant needed, and cheap monthly payments.
However, if you want to do things legally, by far the biggest headache is the fact that you are obliged to bring all the money you earn from your foreign clients into Argentina within 5 days of receiving the payment.
At the moment, you can receive the money in PayPal, but you cannot leave it there or do what you want with it. You have to 'liquidar' the money by transferring it to your Argentinian bank account. The only way you can do that right now, as far as I'm aware, is to use a third party platform called Nubi.
If you get paid into a foreign bank account, you have to transfer the money into your Argentinian account via SWIFT within 5 days, fill out forms in the banks, etc. It's a huge hassle.
Will AFIP catch you if you leave your money in PayPal? Who knows. But basically, if you are legally declaring your earnings, you will have to fill in a Factura E invoice for every client who pays you. That is then on the system, and the money you earn has to be brought into the country. It also can't be brought in by a service like Western Union, it has to come in via what's called the MULC
Lots of people don't declare all their foreign income. You could do the same, just declare some of your payments coming into PayPal, transfer those in, use that for your Monotributo, then keep the rest somewhere else. Just make sure you know what you are doing before you start doing that.
Feel free to PM me with any questions if you want. I've been through it all before and it can be very confusing.