Apparently. most of the applicants experienced the same confusion, and I am no exception.
Two days after I sent my sponsorship and OWP application, I noticed the lack of payment. As soon as I noticed I only paid the fee of open work permit holder and did not pay work permit fee, I made it up online and saved the digital receipt. Then I uploaded the receipt to IRCC via IRCC Web Form. In the message I mentioned:
CIC receipt number
Canada Post tracking number
Name
Date of Birth
Address
Type of Application: Application for spouses or common-law partners (currently living in Canada) applying under the Spouse or Common-Law Partner in Canada Class
Then 5 days later, IRCC email back saying they have forwarded the information I provided to the responsible department. They will contact me if they require further information.
I sent my application on 11/15 2017, AOR received on 01/05/2018.
So I strongly suggest you use IRCC Web From to upload your receipt immediately. Good luck.