Invoices that are posted from other systems need to include a new field called "onlinepaymentmethod".
This API Help topic refers: AccountRight 2018.2 | release notes – Support for the MYOB family of SME product APIs
The field's value should be set to "None", because it now defaults to "All", which is why your invoices include online payments as an option.
You should refer this request to the vendor of the other system. If they elect to support this feature, it may be necessary for them to provide a new option in their configuration settings.