I have found where the economy baggage allowance has been 20kg then the check in staff will usually allow ~5kg to travel for free. Once you start going significantly above the 20kg limit, then you'll be charged excess baggage.
The airlines I am talking about are BA and QF.
Note, your behaviour at check in will determine how you're treated.
It's important to weigh up sending stuff unaccompanied versus excess. I can't remember the unaccompanied rates but you do have to factor in returning to the airport on another day and possibly paying customs fees. Excess baggage (from memory) costs around A$30/kg.