Amazon doesn't inform users when the year is up and your card is being charged again (unless the card has expired or some other issue).
They've become pretty good, however, at tracking/surveilling their users to "improve services" and spam your inbox with product offerings but they can't manage to let you know when charging you for Prime.
In contrast, I use a prepaid cell service that I set up to charge monthly. The day before a CC charge, I get a SMS message informing me that my card will be charged. The next day, I get a confirmation SMS about the results.
Also strange... it also states on the left-hand panel (as of 27apr18, 1pm PDT):
> Next Payment: On September 25, 2018, you will be charged $99.00 for a year of Prime.
From all the articles I've read regarding this, the cut-off date for existing prime customers is June 16th. If I'm to be charged $119 on Sept2018, it should already be noted on the Prime Pages. Seems like a user-hostile action on Amazon's part.