Amanda Lepore is all for Hunter Schafer's decision to have bottom surgery ... since it's allowing her to live her life authentically.

We caught up with the trans model and fashion icon at LAX Tuesday, who tells TMZ ... everyone's got a right to be happy, and sometimes surgery's part of it.

Check out the clip ... David LaChapelle's muse even offered up some advice based on her own experience, giving Hunter a few tips for recovery.

Amanda tells us that she had her own surgery a long time ago -- joking that she's made good use out of the procedure.

She tells us Hunter's going to have so much fun ... as long as she handles the aftercare properly.