On the safer side i also paid OWP fee: 155 CAD and submitted the receipt by Web Form with the following explaination letter:
Dear Officer,
My name is XXXXXX. I am in Canada on Visitor Visa and married to a Canadian Citizen.
My husband has applied for my sponsorship under Spouse or Common Law Partner in Canada Class which includes my applications for Permanent Residence and Open work permit in one package and received by CPC-Mississauga, P.O Box 5040, Station B, Mississauga, Ontario, L5A 3A4 on August 8, 2018.
While paying application fee for open work permit, I paid only for Open work permit holder (100 CAD) but forgot to pay Work permit fee (155 CAD).
I am afraid that due to insufficient funds my application might be returned so I am paying Work permit fee (155 CAD) now.
Payment receipt is attached with this letter.
I will appreciate if you can attach this fee receipt with my application for open work permit.
Thank you for your understanding.
You can reach me on cell: XXXXX or email: XXXXXX
Yours Sincerely,
XXXXXXX
First Name: XXXX
Last Name: XXXXX
Nationality: XXXXX
Indian Passport No: XXXXXX
Address: XXXXX
Hopefully it works.