Is it possible to create a paid application which makes use of an Automator service?
Yes, it is definitely possible.
I don't think the logic that it is necessary that a paid application is opaque and static is true. It is entirely possible to sell software while providing access to the source code and letting the customer modify it. Software has been sold that way by large and small companies for decades.
If you really want a technical solution, you can use obfuscation to hide the plain text of scripting languages such as AppleScript. Some languages have an eval-like functionality enabling you to deobfuscate source code and run it directly, whereas for AppleScript you probably want to store it in a seperate object file and run it - or employ OSAKit to be able to run the source directly.
Most programmers in closed-souce, commercial software would probably not employ such &quot;tricks&quot;, but rather go for a compilable language such as Swift, Objective-C, C++ or similar to get an executeable that is not immediately &quot;readable&quot; by humans.
Note that all software that can be run can also be reverse-engineered.