There is a limit to the duty free items you can bring into Mexico, but its been so long ago that I don't remember if its $300 or not. Sounds about right though. As you know it only applies to items you intend to leave. One way around it is to fake receipts so that you are under the limit. Another way is to just say that you will be taking it back home with you. Easy if it is a laptop, but hard to explain if it is a couple NIB Ipods.
We always take things to friends when we travel to Mexico, just not in case lots, and have never had a problem.