Managed to solve it by buying the game over mobile data instead of WIFI. Posting some of the solutions suggested by support here for anyone that needs it:
- Try a different browser
- Delete cache/cookies/browsing data from browser
- Try on a smartphone with data instead of WiFi
- Try on a different computer
- Try from a different location
- Checking out as a guest (for purchasing an order)
- Disable any VPNs