Just a quick update: I just bought a 15 euro topup from the folks below. It was $24.50 AUD. Method of payment was paypal. I received the code immediately right there on the screen, and also via email. Total in US Dollars was $17.83.
Very efficient service!
http://www.riiing.com.au/Index.html
Phones: Xiaomi Mi Mix 2, Samsung Galaxy A50, ASUS zenfone 3,
Sim cards: AT&T (Contract), 3 UK, Piranha Mobile