I don't want to go off-topic, but just to clarify:

In Argentina, credit cards don't transfer the money to the merchant right away, but rather it takes 15-20 days for the money to be transferred from the CC into the merchant's bank account. It's an old an problematic issue, mostly due to lack of regulation and zero competition in the credit card business (a pool of large banks owns the one company that operates the Visa and Mastercard brands here, and Amex, while independent, plays their game). This is why you'll see many stores (particularly the smaller ones that only cater to locals) accepting debit but not credit cards -- debit cards transfer the money in 48 hours or so. The Central Bank has been taking measures to address this issue, and over the last few months I have noted many small stores in BA now accept CCs under the Mercado Pago scheme, which is Argentina's version of PayPal. According to a friend who owns a small wine store, Mercado Pago transfers him the money in 7 days instead of 15-20, and the commission is lower.

Under this scenario, the TA will get OP's money, converted into pesos, 15-20 days after OP paid. I am not really familiar with how exchange rates work in those cases, but from what I read I understand she is afraid the CC will convert dollars into pesos at the rate of the day OP paid, instead of the rate of the day she got the money. I can see what she means, but I just can't understand why she won't take a small deposit through CC and ask OP to pay cash upfront once he gets to Argentina.

That said, I agree with David Beach. There is no hidden or geheimnis Uruguay. It's a small, straightforward country. Maybe OP can tell us what the TA was offering him and we can help finding the way to book the same services without the TA.