Preparing your file for Carbon Printing:
Grayscale image - by going to Image/Mode/Grayscale
8 bit - by going to Image/Mode/8 Bit
2.2 Gamma - by going to Edit/Convert to Profile/Gray Gamma 2.2
Files have to be saved as FLATTENED tiff files.
When you upload their files, just record "CARBON" where the paper naming convention normally is in the file name.