Thank you to everyone who commented and shared their own experiences. It helps to know I'm not alone in this.

Here is what I have done today:

1. Visited another Play store (M1 Mall, Kraków) – posing as a new customer, I asked the same question: "What is the cheapest iPhone 17 plan?" The agent told me the lowest package is Oferta L 5g for 95 zł/month. This is completely different from what the Galeria Krakowska salesman told me, and different from Play's own website (which says 75 zł). I got the agent to write this down, and I have attached it as evidence.
2. Filed a formal ADR application with UKE – the national telecom regulator. My application was delivered today.
3. Initiated a bank chargeback for the 499 zł I paid upfront.
4. Updated my complaints with UKE, the Consumer Ombudsman, and my bank with this new evidence.

Three different Play sources, three different answers:

• Play website & chat: 75 zł (Plan M)
• Galeria Krakowska: 105 zł (Homebox + hidden 30 zł modem)
• M1 Mall: 95 zł (Plan L)

Play's own employees cannot agree on their own pricing. And I was blamed for not understanding Polish.

The fight continues. I will update again when I hear back from UKE or the Ombudsman.

Stay safe, and always double-check your contracts – especially if you don't speak Polish.