It doesn't all end up in Customs at Ezeiza (for FedEx and DHL - other items, like USPS 1st class goes to Correo Argentina in Retiro, for example, not nearly as bad). For sure, smaller envelopes with papers never get held up there and will make it to your front door.
I'm not saying a hearing aid will get passed through, but I get ATM cards and other small items like that all the time. The only time my packages get held up is when I get boxes that have a lot of stuff in it. I have a mail forwarding account in the States and get my mail via DHL every month with no problem as long as I don't send bulky items down. I got a CPU a couple of years ago with no problem like that (taken out of its box of course).
However, with your particular issue, I'm not so sure. If you have to declare it in the paperwork for insurance purposes, there is a good chance it may get held up. If whoever is returning it to you can declare it as an inexpensive item and place it in a letter-sized envelope I don't think you'll have problems. But as soon as you have to declare exactly what it is, it will probably get held up in Customs (at Ezeiza with "overnight" couriers, at Retiro with first class and registered mail). You may or may not have issues with taxes paid at that point, depending on whether or not you could show it was sent for repairs and is not something brand new.