Fax payment receipt to IRCC for underpayment?

[stuent97]

So I submitted/mailed my pgwp extension application (due to early passport expiry), and I submitted $240 with receipt included, today I received an email from case processing center Edmonton saying I did not pay enough and I have to pay another $100. In addition, the email ask me to fax the receipt to Agent DA/RTR with name and UCI inidated on front page

My quesiton is :

1. Did anyone have similar experience before and how did you include your name and UCI with the receipt? Did you just print another page having name and UCI (maybe also application no)?
2. What is this Agent DA/RTR, does this refer to some sepecific department?

3. Also if I linked the paper appication to my account, should I see all the document I submitted? Currently I am not able to see any submitted document under the document section.

Some info regarding the email:
1. The email address ends with cic.gc.ca
2. The email contains a biometric letter with my UCI and application Id
3. I tried to link the applied and it go through. The application Id is the same as what is in the biometric