An SSID is a security measure, the software providers have gone through an extensive security audit to be permitted to submit STP data to the ATO. The SSID is created by the software provider and thus it would not be possible to submit STP reports without an SSID as this would undermine the security in place.
Submitting STP data means the business has staff, thus I think the argument would be if the business can afford to have staff they can afford the software to submit STP data.
Microkeeper for example offers a free plan for employers with a single employee, thus hopefully you can find software solutions that are affordable.
Another thing to keep in mind is the software fees usually offset other costs, for example the software might cost $25 per month but save several hours per month in bookkeeping fees thus the business is better off.