I don't believe a PO Box is mandatory. You can simply provide your current residence/mailing address.
Below is a list of what you'd need in general, at the ITA stage:
1. Medical report from an approved panel physician
2. IELTS results
3. Police reports from all countries that you have lived in for 6 months or more. You can use FBIReport.com if you're a US citizen; this is faster and preferred by most. Otherwise, you'd have to submit a conventional FBI report request, which takes a few months.
4. College transkripts/certificates with degree(s) endorsed by WES or similar.
5. Employment verification letters from current and past employers
6. Recent pay stubs, W2s/T4s, depending on where you've worked
7. Birth certificate, Marriage certificate (if you have spouse to include in the application)
8. Letter of explanation - this is optional
9. CIC application fees