The concept behind core charge is simple.
Let's say an Alternator costs $200 with "core charge" of $100.
So you get the Alternator provided you return the old Alternator so they can rebuild it and re-sell it later, making money.
Let's say you fail to return the old Alternator, the seller will charge you $100 on your VISA Card for failure to return the core, now your cost is $300 for failing to return the core.
However, if you return the core as instructed, then it would cost you $25 for the shipping; so your total cost to get a rebuilt Alternator is $225.