Never, ever listen to anyone at Home Depot/Menards/Lowes, etc. Nothing but a bunch of idiots.
If you are going to make your own cord, get some 12-3 SO cord. It will probably be about the same price as the extension cord, but will be much more durable.
I like to squeeze the nickle until the buffalo craps-mt rob
"Why don't we get drunk and screw?" Jimmy Buffett