Official comment
With the Pro subscription you get processing for up to 20 maps per month and each of those maps can contain up to 3 gigapixels of source imagery without further charge. You would need to pay for any additional gigapixels using the pay-as-you-go points system as needed.
595 twenty megapixel images is 11.9 gigapixels but if you use the .25 downscaling option in Step 2 of the uploader that would get the total gigapixels down below the 3 gigapixel threshold so you wouldn't have to pay anything additionally.
If you were to process those images without the downscaling you would have to pay to process the additional 9.9 gigapixels at a rate of 200 points per gigapixel. Since you are a subscriber there would also be a 20% discount applied to the point charge since the job total is below 12 gigapixels.
