You can pay the fees at the BLS centre. It would be ~$10 extra.
Once you completed the parts, first upload the photo and signature. A scanned image with the required solutions/ pixels can be attained by using the MS Paint software.
For other documents, you need to select the drop-down and upload the docs in PDF format. I normally use the free foxit pdf to convert images/ docs to PDF format. While converting the docs, use a lower resolution 96 dpi or 144 dpi as the total size should be below 500kb. For multiple docs, you can use an online pdf merge utility to upload.
Basically, you need to upload you child's birth certificate, passport, plus you and your wife's Indian passports, COPR, PR, marriage certificate in the appropriate drop-down menu.
Even if you forgot to upload 1-2 docs, just note down in a sheet of paper as to what docs you had uploaded and the BLS people will give you what else to be uploaded. They will accept the forms and collect the fees; but will remind you to upload the remaining docs asap.
For the BLS office, you can go to the nearest one and their website give some info on procedure.
