Amazon is offering a $20 shopping credit if you sign up to Prime via Unidays.
Works on the monthly payment plan, or the yearly plan.(Confirmed by Doweyy)
The TnC's states "eligible items".
"For the purpose of these terms and conditions "eligible item(s)" refers to any product sold and shipped by Amazon AU on Amazon.com.au or Amazon app and does not apply to purchases made from third party sellers (including Amazon US and Amazon UK) even if fulfilled by Amazon AU, digital purchases such as e-books, subscriptions such as Prime Video and Audible, gift cards and postage or delivery rates."
You must not have been a Prime member 30 days prior to February 21, 2022 to be eligible.
Full terms and conditions of the deal below:
Can confirm it works for monthly plan and is provided instantly.