Ok, I think the issue is what "Response Method:" is set to within your moneris account. Once logged in navigate to Admin > hosted config then edit the hosted config you're currently using. Next change "Response Method:" to "Sent to your server as a POST containing XML". If it's already set to that try using "Sent to your server as a POST". See the below configuration instructions.
ADMIN > hosted config > Generate a Version3 Configuration > Basic Configuration
- Copy "ps_store_id" and "hpp_key" respectively to gateway
- Apply gateway to generate URLs needed later
- Transaction Type: Enabled
- Response Method: POST (with or without XML)
- Approved URL: (generated in gateway settings)
- Declined URL: (generated in gateway settings)
- Save Changes
ADMIN > hosted config > Generate a Version3 Configuration > Paypage Appearance
- Check "Display item details."
- Check "Display customer details. (cust_id, email, note . . .)"
- Check "Display shipping address details."
- Check "Display billing address details."
- Check "Enable input of Billing, Shipping, and extra data fields on the hosted paypage."
- Cancel Button URL: (generated in gateway settings)
- Save Apprearance Settings
ADMIN > hosted config > Generate a Version3 Configuration > Response/Receipt Data
- Check "Return line item details."
- Check "Return shipping details."
- Check "Return billing details."
- Check "Return other customer fields. (cust_id, client_email, note . . .)"
- Check "Return ECI value."
- Check "Return the txn_number. This field is used to perform follow-ons."
- Check "Perform asyncronous data post."
- Async Response URL: (generated in gateway settings)
- Save Response Settings
ADMIN > hosted config > Generate a Version3 Configuration > Security Features
- Check "Enable Transaction Verification"
- Check "Displayed as XML on our server."
- Response URL: (generated in gateway settings)
- Save Verification Settings