@Bio_HUM Thank you for your response! I never got an e-mail notification about your response so my apologies for the later response.
@notalurkeranymore, sorry for not updating my post; have not been on here for a while. So yes, in fact you can upload your statement of intent after having paid the fees. I have included a quote of an e-mail communication I had with the NPSIA admissions team
"Yes, you will have until the January 31st deadline to upload all application documents, even after you have submitted your application fee. We will not be reviewing files until the beginning of February, so as long as the documents are uploaded by the 31st there will be no issue. For more information on uploading additional documents after you have submitted an application, please see this link:https://graduate.carleton.ca/apply-online/required-documents/"