(singsongy voice) He's trying to rip you off.
Galvanized will not last 2 years, but it costs a small fraction of what SS does. For fasteners/threaded couplings, you want 316 too. Don't buy the old "but 304 is stronger" line - although it's true, 304 will sieze up in those acidic conditions. You'll never remove/decouple them.
Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve