EFS-Web is not very user friendly. You probably already looked at the USPTO's guide to EFS-Web. I don't think the EBC Help Desk people - who are very patient and helpful, will be much help with actual decisions, more the mechanics of uploading.
The PDF files need to be 8.5" by 11" (or A4) and have names that are EFS-Web legal. If a PDF is somehow 8.25" by 11", for example, EFS will complain. Embedded/not embedded fonts can be an issue. Print to PDF can eliminate this. That can also help if you have mathematical symbols. Saving an MS Word document to PDF will not always reproduce math symbols correctly. It is possible that your drawing files will be too large.
You should download and fill out an ADS form in advance. They will probably not be able to help you decide to file as a small entity or a micro entity. A PO box can be your mailing address but there is another line for the city of your residence. This needs to be the city of your residence.
You need to decide who the inventors are and fill that out on the Declaration. It needs to be signed. If it is not signed it doesn't, bureaucratically, exist. If you have not already come across S-Signature, you might get comfortable with how the USPTO allows you to sign things with their "/XXXXX YYYYY/" format.
It is not required at the time of filing, but it might be a good idea to fill out an IDS form and file it with the submission.
Place all of the files to be uploaded in a folder by themselves, again, with legal names.
Very definitely, record the electronically generated application number the system gives you at the end of a successful submission.. Without it you will have a hard time learning about its status later.
If you do not already have a USPTO Customer Number, I would ask them to help you get one. Without it you can't follow the status of your application on-line. I like to file, at least, a couple of days before a deadline. Then, later that day or the next day, I can look in Private PAIR and see if what I thought I uploaded is really what is there. Anyone can get a Customer Number - without your application being associated with one, you can't see it.