Although Sir Cooper allows for a higher DPI for print and has no minimums they don't allow "Full Bleeds" (printing to the edge of the paper) and require a 1/4" border. Also, I couldn't find a mention of what paper they use for Poster prints. It looks like they do simple Color Printing on a color copier (still good quality) and not Photo printing. It may be what you are looking for but thought I'd point those things out from what I saw looking over their site.
Twin