Report 90 days question.

I'm well aware that 90 days online report is not working (additionally, security certificate on IMM website expired by the end of February). So, I done my 90 days as usual for me - by postal mail, almost 3,5 weeks ago, report sent by EMS and delivery was confirmed. However, I not yet get receipt back. And I will need to show that receipt at CW to get yearly extension. So, as I see I have only one option - in case I not get receipt back by mail by the end of the month, I will need to take my usual documents, proof of postal delivery, copy of old 90 days (original was mailed to CW), then go to MTT and 'claim' that receipt before I visit CW for extension. Please advise if that correct or not.