If the shower pan is CPE, and it almost certainly is, then it will
probably never leak. This is especially true if there's no seam.
You will eventually have a problem with mold and discoloration,
Send a certified letter spelling out your expectations and give them a
reasonable deadline to hear back from them. If they still try to avoid
you, go to small claims court and file.
Filing fees are generally less than $40, and just initiating the
process is often enough to spur them into action. I don't imagine a
Judge will side against you, but make sure your ducks are in a row.
AFTER they've fixed it, contact your local licensing authority, the
BBB, and anyone else you can think of (Union?). Avoiding a disgruntled
customer is a terrible way to do business.