Actually, there is evidence here:
Specifically this section:
CCP offers a variety of PLEX packages from the Account Management pages, but players wishing to purchase 150,000 or more PLEX at once should send an e-mail to PLEXSales@ccpgames.com for further information on bulk purchases
It is right at the bottom of that page, right above “Attachments”. They’ve had similar notices for sometime, years in fact, if I remember correctly.
I’ve never made a “bulk” purchase; but CCP does advertise it.