My builder sent out a slip that said the put bullseye shellac on my mahogany
front door every year to keep it looking its best.

Shellac is a pretty weak finish for use. I'm was thinking more like
Helmsman spar polyurethane to hold up to the elements.

The top and bottom of the door are not finished. Should I seal those or
leave them open to let the wood "breath".?

What would you guys recommend for finishing a mahogany exterior door.