You should be able to access ECAS now with your receipt numbers, but it won't say anything, it will say that it can't find your details on the system, or something like that. They only update ECAS once they have started processing your file, literally opening it up and that will only happen after the 'current processing time', see here
http://www.cic.gc.ca/english/information/times/canada/process-in.asp#perm_res. In other words, if it has 43 days processing time in Mississauga, then you will (likely) have to wait 43 days after it was delivered on 24 August, to see ANYTHING (about your file) updated on Ecas.
I'm not sure how your payments came to only $300 though? You came up with a very odd number there, it should be more like $1,040, breaking that down: $75 for the application, £490 for the PR and $475 for the applicant, non?? Nowhere near $300 I'm afraid but I could stand corrected....?
Fees calculator is here
https://services3.cic.gc.ca/efee/efee.do?lang=en