I use the phone number to make the reservaion and was given a booking numbers. No credit card number was give so I guess I pay once entering the building?? This is all new to me.
Great start so far. For the Accademia, ther will a separate line for groups, reservations, and walk-ups. The group and reservation line use tha same door. Arrive early (15 mins), but expect to wait anyway. You'll pay once inside, show the cashier your reservation number. You can rent an audio guide with an i.d. as deposit.
For the Uffizi, if you're approaching from Palazzo Vecchio, immediately proceed to the reservation entrance on the far right and pay for your entrance. Should be listed as door #1 (I think). the reservations entrance is across the way to your left (however that can change at anytime depending on the crowds. Audioguides can be rented with a Photo i.d. just before entering the gallery.
At either location, if you're with a partner and you both want an audio guide, just rent for one and bring your own pair of earbuds. Just plug them into the rented audio guide and listen in. If you rent for two, all you'll get is an audio guide and an extra set of headphones anyway.
We just paid the pittance fee and booked online with a CC. I feel better having done that personally.
We only had the booking numbers as well - and it worked great. Just be prepared to pay cash only and show up with your number. They look you up and you are in. No reason to pay fully in advance. It worked nice for us because we ended up reserving, unknowlingly, on a free entrance day - so we only had to pay the 3 euro booking fee.
Just make sure you go get your tickets maybe a half hour beforehand. They recommend 10 or 15 minutes before the time you're to be admitted, but we did that and after getting our tickets, we stood in the line to enter the museum for about 45 minutes. Also, in case you're interested in audio guides: I'd go for an audio guide for the Uffizi - skip the Accademia, it felt like a waste of money and didn't really tell us anything.