Sorry if any of this repeats what's already been said, but I brought my cat to Bs. As. last month, so my experience is still fresh in my mind. I'll qualify all of this by saying that, again, I brought a cat and not a dog. That said, some points below might be helpful.
First and foremost, what I'm thankful I did was go to the USDA office in person. Fortunately, the location in Miami was only about 40 minutes from my parents' place, which is where I stopped off before heading to Bs. As. I called the USDA and made an appointment. The main benefit of going in person (aside from getting the form stamped and handed back to you within about 10 minutes) is that if your vet f'd up the form you have to take with you, you'll know immediately and will have time to sort it out before you're supposed to leave. The USDA offers a service where you mail the documents to them (and enclose a return envelope), but I didn't want to risk it. Aside from the USPS or FedEx/UPS losing the envelope, if something had been wrong with my forms I wouldn't have been able to get what I needed in time (the 10 days before rule is a killer). In the end, my vet rocked the house and got everything right, but I was still glad to have been at the USDA office in person.
Secondly, and this wasn't an issue for me (even though I think I technically didn't follow the rules), allegedly ARG requires that the pet have the rabies vaccine 30+ days before you travel. Fortunately, SENASA didn't enforce its own rules (maybe because I got to EZE at 10pm and the guy who helped me seemed totally chill) because I learned about this "rule" only after I got all the paperwork stamped at the USDA — and the ARG Consulate in Miami confirmed it. Sometimes the lax attitude here works in your favor
Contrary to what someone else said, I opted not to buy the forms online. The vet had the form I needed. That said, you might want to call your vet to make sure he/she has the
APHIS 7001 form. Don't waste your time printing it out and bringing it to the vet; turns out that the form needs to be in triplicate. Not even sure why the USDA provides it on their site.
Finally, as others have said, SENASA will be more than happy to take your cash when you get here. I think the fee was about 330 pesos. You go to the SENASA line*, the dude or dudette will fill out a form, ask for the USDA-endorsed form (and possibly ask to see the rabies vaccine), and then send you next door to pay the fee. Once you get the receipt showing you paid, you head back to SENASA, show the receipt, and you're good to go.
All in all, it was a fairly painless process. Things flowed smoothly at MIA and EZE...and it was all made possible because the vet I used was awesome and the USDA applied that nice little seal to the health certificate...and took my $37 in greenbacks.
Hope this helps. PM me if you have any questions.
*Most people at the airport didn't seem to know what I was talking about — so as an FYI, SENASA is in the same area where you go through Customs (where they scan your bags).