I had my first experience with uploading files for printing at Costco yesterday. My local Costco just went online last week. They use a Noritsu 3100 series that can print up to 12x18 on Fuji Crystal Archive paper at a cost of $2.99. That also means you can gang print 8x10s for a print price of $1.50 instead of their $1.99 price. Normally I bring in TIF files on a CD but the upload only can take JPEGs, and it goes through Snapfish.
I had a 12x18 prepared on a CD so I converted it to a maximum quality JPEG of 8.8 megabytes and uploaded it and specified which store I was going to pick it up at. That went smooth enough but you have to be aware of the options before finalizing the order. By default it has "auto correct" set and specifies glossy paper. It also has has one 4x6 ordered for each print which you have to uncheck when specifying larger sizes.
I also took the CD to have the TIF file (56 megabytes) printed to compare. Never having previously printing from a JPEG, I was curious to see if there was a noticable difference. There was a slight difference in that the tonal range and contrast looked slightly better from the TIF, to the point where I won't be printing 12x18s from JPEGs. And since I don't do individual 8x10's, I guess their upload service isn't for me.