Just to share my experience.
To buy the wakuwari (20% discount) main entry tickets, you can only do so through USJ's japanese website.
Like what
idiot444 has mentioned, you could consider buying just the main entry through the japanese website (with signing up a Club USJ membership with Japanese inputs for name, address and etc) and then the express pass through the USJ's english website.
However, there are some issues with that.
1. USJ's japanese website is unable to process overseas credit card. I've tried my Revolut and various banks' CC and can't complete the transaction.
2. USJ's english website does not have all the variety of Express Passes that its japanese site offers. Good if what you want is available on english website. I suspect that the timed entry combinations are limited too on the english website. just guessing.
From my online search, it seems like there are foreign travellers (on reddit) who were able to complete their purchases with Apple Pay. However, when I attempted to try it, I found out that Apple Pay option is only available on USJ's english website, and not on the japanese website. Based on the reddit comments and what I've experienced,
it leads to my conclusion that likely even USJ's english website may not be able to process overseas credit card. (I stopped at the payment page. Did not follow through the USJ english website with Apple Pay)
Also, just to highlight, in my google search results with the error codes from the failed process, some travellers (on some japanese page; likely domestic travellers) called their bank to "loosen" the security. I tried to contact Revolut to see if there is anything they could do, which I was told there is nothing for them to do, as there is not even a transaction rejection or attempt to Revolut. With this, I conclude that the comments in this case does not apply to international buyers.
Conclusion
The optimum priced and flexible tickets option is on USJ japanese website and you are
unable to buy it online without a japanese credit card.
With all that, I still managed to buy my USJ tickets with Express Passes, through the USJ japanese website (coz I'm desperate for the 20% wakuwari discount and the flex in Express Passes choices).
What I did was at the
japanese payment page, I
selected the convenience store payment and managed to get help from a friend who was travelling in Japan, to
make the payment at the convenience store in-person (7-11 specifically, as only this konbini allows credit card payment) within 2 days of the tickets purchase (deadline is given and will be stated after completing the purchase on USJ japanese website).
The QR codes for the tickets will be released to you after payment is made.
Sorry but the solution presented may not be ideal but I could not find any better alternatives to the best outcome.
Maybe if any bros in HWZ who are already there in Japan now, can help the bros here? lol
Alternatives
Another alternative is Klook. Their main entry tickets are more expensive (no wakuwari discount option; even normal pass tickets are more expensive than what was sold on USJ website). Their express passes surprisingly is slightly cheaper (few dollars) than what was sold on USJ japanese website, however, it is limited in variety options and you are unable to choose your desired timed entry combinations (supposedly randomly allocated as indicated in their T&Cs).