If your products are all "virtual" (like downloads), then the built-in Zen Cart processing will bypass the "shipping" step in the checkout process.
I realize that's not a full answer to your question, but you could use the referenced plugin's v1.0.x implementation as a guide for changes to your ZC1.5.0 store.
<nag>You should really be thinking about upgrading first and adding new features later; ZC1.5.0 stores are, IMO, a disaster waiting to happen as more and more webhosts are dropping support for PHP versions prior to 5.6.</nag>
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