I often use #140 watercolor paper. Although I stretch before beginning work, oftentimes the final is not completely flat. I have been doing some research on this common problem and can offer a solution. These images show a before and after using a badly warped sheet of #140 Arches.
I carefully iron the back side with a steam iron, protecting the paper with a completely flat clean bedsheet. Gently rub the iron back and forth, covering the entire surface.
I then stack a large heavy stack of books (coffee table type books) and let the piece completely cool.
If you take to Michaels or similar, they will not take out any ripples that may have occurred. Take the time to check over your piece before framing and follow the above procedure before you take to the framer.