Shipping is an issue. I avoid shipping anything with glass. It's just too easy to break but Plexiglass is another story. I would not put plexi on a pastel however.
I think artists underestimate the power of their opinion when selling art. It's a, "package deal," it comes framed. What are you supposed to do with a custom frame just eat the cost? Just because they say they don't like the frame doesn't mean anything. It means they are haggling on the price or they wish to treat it as some decorative piece of fluff and frame it to match the couch, bad attitude.
I get folks in my frame shop all the time that wish to keep a cheap frame because the, "artist framed it that way." So it must be the best/right way to do it rational. Of course they may be just unwilling to pay to frame it correctly. If you refuse to haggle with them (sell it without the frame) by selling it in pieces where does it end? The conversation has shifted from selling something of value to a simple commodity. The reality as often as not is that they believe they can frame it for less and or they ascribe to the theory, "if you don't ask, you don't get." They are most probably going to spend just as much or more to frame it with the same frame and if they do frame it for less it will be in an unflattering cheap frame. Even in my shop where changing a frame is easier, what am I supposed to do with a custom frame designed specifically for that image? The odds of me selling it are minuscule. You are colluding with the mass production throw away mentality and devaluing your work in the process in my opinion.
They like what they SEE and in part that is the way you have framed it, they just want to haggle. Will you lose a sale sure once in a while? You are the expert however they are likely not. If you get confused with framing choices and you are the artist, think how overwhelmed they are by those same decisions. They only know what they like and sometimes they don't even know that. They know what a so called expert tells them.
Discount it maybe if they buy two or more but it comes framed. Would you agree to sell your car and haggle over the wheels? No it comes, "as is"