Sounds like a month-long project. In reality — one coffee break.
Copy your API token from Poster, paste it into settings. Client sync starts automatically. Pick your cashback tiers (3%, 5%, 7%), upload your logo, choose your colors. Print one QR code for the counter.
Customer scans it, registers in 30 seconds, gets a card in Wallet. Turn on automations — welcome messages for newcomers, reminders for those who haven't been in a while, birthday bonuses.
From signup to a working program — under 5 minutes. Seriously.