Uffizi Annual Pass

Can't help you with buying the passes thru the website but would advise that buying them upon arrival is pretty straightforward. We simply showed up at door #2, told the security guy that we wanted to purchase the Amici Degli Uffizi cards, and were hustled into an office just inside the door to complete the purchase. The whole transaction took about 15 minutes.
The cards essentially gave us head of the line privileges - ie no need to book a specific time - we simply showed up at the door, flashed the Amici cards, and were granted entrance with the next group waiting to enter after a short wait. You still have to go thru security but it was a major time saver for us - plus it allowed us to visit several times for shorter periods.

We bought ours a number of years ago at the Uffizi Welcome Desk because it looked to be easiest that way. I personally am not crazy about Paypal or bank transfers if I can avoid it.

Editing to add, I see Robert bought the same way. :O)

http://www.amicidegliuffizi.it/diventa_socio.php?pg=8&ln=en

Are you trying to use the B-ticket site? If so, I don't think you can buy it through that one.

As Robert mentioned above, it is best to purchase upon arrival. They will need your passports as they associate each card with a specific name. Our family is utilizing it more frequently for Boboli Garden as the weather gets nicer : )

I purchased an annual pass Uffizi pass a few months ago through the B ticket site and had no problem. However,
my first attempts at purchasing a ticket for the Academia through B ticket were problematic. After a week or so I
tried again and had no problem. Also, this is the time of year when opening/closing times and days change as the
heavy tourist season is beginning, and some sites are updating. For example, the Bargello site was under renovation last week. Give it another try.