Just completed the checkout, but Drive still shows 15gig... does it take a while before the extra gigs are added?Think so yes, mine showed 15GB right after as well, but today it says 17GB.
I've done something like 10 accounts so far, the longest any of them has taken to show up the change is 3 minutes - just sit in GMail and refresh the page every minute, it tells you how much you have down the bottom of the email list, eg. Using xx of 17GB (or more, it's cumulative, I have 19GB on a couple of accounts).
I don't get the Upgrade Storage button either, just get this:2GB additional Google Drive storage for free
The 2GB was added by the time I switched to the other tab to check.
EDIT: That's just from logging into https://myaccount.google.com