Patrick,
Take the telegram you received from the post office to Aduanas where you will pay the charge for your package. I had the same problem -- no local bank account to make advance payment. It doesn't matter. You'll end up paying the DECLARED VALUE of the item in pesos. It's a ripoff because they get away with it.
A friend sent chocolate from Scotland that I never picked up. A magazine sent me a free DVD from the US that I ended up paying 100 pesos for (the value). I have told friends not to send me anything. The only packages that the post office delivers are magazines.
Prepare yourself with a good book and water. You will be waiting hours at the post office as the process moves very slowly. Hundreds of people are in the same situation.
Janis