This afternoon, I get a phone call. "____ bike shop in town referred me to you."
Ok, what's up?
"I need a really long seat thing."
"Yeah. It needs to be 20 inches long."
Quick calculating in head... wow, 500mm? What kind of bike is it, a folder?
"It's a Mongoose. Anyway, it needs a 20 inch seatpost. Maybe longer."
Dude, your bike does not fit. You need a different bike.
"No, seriously, I need a 20 inch seatpost...how much would it cost"