Most all hotels in Vegas now charge a handling fee for receiving packages, and $6 is on the cheap side.

As far as warning people in advance that this fee exists, it is IMHO, your responsibility to ask if there is a fee, not the other way around. Are they supposed to run through a list of possible fee's everytime someone calls and makes a reservation? BTW, there is a fee for using the phone for a long distance call, a fee for laundry service, a fee for receiving or sending faxes, a fee for our internet, a fee for your 50lb cat, a fee for.....

I probably would have just mentioned that you have an package, and discussed the fee when you picked it up, or just billed it to your room.