For me, I think it's wonderful that you can get a great print or poster for $40. That problem has been solved. But you can tell a cheap frame from a quality one every time.
And, yet, unlike a doorknob, sometimes you want a frame to have a custom size.
I get sticker shock every time I go to buy a nice poster, print, or art piece at what seems like a very reasonable price and then see the cost triple or quadruple when I add the frame and shipping.
You anchored at 40 - and surely the frame is less than the artwork.
Can I suggest a bottom "entry level" solution - it might not work but have cheap(ish) standard sized frames and adjust the inner piece of off white card that goes inside.
I have two kids and would love to swap their work in and out on a weekly basis - different sized card to cover things up seems great.
Maybe YOU "can tell a cheap frame from a quality one every time" but I can't. I've never chosen to develop that expertise. Frames all kind of look the same to me, except that a few styles of wooden ones look needlessly gaudy/ornate/expensive. To me, the purpose of the frame is to FRAME - it's not art, just a square one puts AROUND art to protect and separate it. I care what color that square is and what size it is, but what MATERIAL it's made of...not so much.