Buick 2004 Century Custom

How difficult is it to program a 4 button Remote Control for my Buick 2004 Century Custom?
I have one for my wife but would like to have one for myself.
I can buy one for $40 on the Internet, but it must be programmed. They supply instructions on "how to program".
Local Buick dealer wants over $100 for Remote unit (maybe $180), plus $40 more to program.
Anyone reading this that has actually programmed a 4 button Remote Control?
Buttons are "Lock", "Unlock", "Unlock trunk", and "Beeper to Locate car in big parking lot".
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