Just following up on my - Vistaprint - 36" x 48" Poster Snap Frames experience - which I was looking at for large Argentine posters.
Long story short Vistaprint customer service were first class and I will deal with them again but shipping these large frames proved problematic.
I ordered two frames (about $40 each after discounts/shipping/tax etc) and they both arrived with the same issue, a bend\dent in the frames.
I don't know if this size flat package can be shipped\handled without causing this damage.

One of the frames also had a severely scratched plastic cover that goes over the poster.
I managed to straighten out the other frame and although not perfect I could get one of my Argentine posters (29x43in) in it to get an idea of sizing.

I was really happy with the 'fit' and believe I could add some background paper or Mat board, as well as potentially replace the plastic covering with Maylar and these frames would be ideal for me.
Anyway, after I discussed all this with Vistaprint's customer service (including my liking of the frame itself) they gave me a complete refund for the two frames and in addition have sent me another two frames (freebies), which they hope I will find useful for my Argentine movie posters. I can't fault this company and I am looking forward to receiving the frames. I will make a post here with an update when the additional frames arrive.