Update: I wrote to Unityinfo@Certiport.com & this is what they replied with. Good news! 😊
"The discount will end at the end of October 2020. We will provide testing dates throughout September and October, but the online testint will end at the end of October."
I'm not 100% certain, but I feel confident that we can buy a voucher (using the CWCL discount code) one day, & can schedule the actual exam later. The web page describing the process had the purchase & scheduling as separate steps, done via separate organizations.
So, I agree with Paul about the longer time frame to take the exam after you buy the voucher.
I did the course Mar-May and immediately took up this offer and registered. It's worth noting that the clock starts ticking immediately, so I'd suggest timing your sign ups/purchases.
GMetrix give you 1 month from sign up date (10 exam tests). Certiport give you a more generous 6 months.
That said, I contacted GMetrix and they extended my tests.