5200px | 300dpi | Jpeg Quality 12 Vs. 16000px | 300dpi | Jpeg Quality 9?

Hello all!

Quick questions for the seasoned veterans here on Fine Art America. I've been selling canvas prints of photos for a few years and they've all worked out quite nicely, but recently got a request for a size larger than I've ever had printed. 48" x 36". As I scrolled through the information on file sizes I ran across the link http://fineartamerica.com/printondemand.html, which asks for minimum 12000px for printing that size...but...well...trying to get that into a jpeg at under 25MB limit is...eh...impossible without dropping jpeg quality from full 12 to around 9 or so. Has anyone here had experience printing photographs on canvas at that large size? If so, any recommendations on what/how to prepare the file?

The two files I have prepared are:
a. 5200px at 300dpi at full jpeg quality (12)
b. 16000px at 300dpi at reduced jpeg quality (9)

FAA Moderator, please feel free to jump in here...they will be ordering in the next day or two, so just want to make sure I have a proper file uploaded. I'm getting confused.

Thanks so much in advance, and happy holidays to all!

Humbly,

wasim.