The "shoe box" priority mailer is not a flat rate box, so it's the same price scale as any other standard sized priority mail item.
Priority Mail Rates
1 & 2
Zone 3 Zone 4 Zone 5 Zone 6 Zone 7 Zone 8 1
There are only two flat rate boxes, one being a squarish one and one being a flatter one. The rates on those are $8.95
Box One: 11-7/8 x 3-3/8 x 13-5/8 inches (ideal for garments, board games, books and other relatively thin items).
Box Two: 11 x 8-1/2 x 5-1/2 inches (perfect for shoes, model cars, and taller items).
We use click and ship almost exclusively for our US packages. The service attaches to your credit card. You will need to use a fairly accurate scale for weights though. We have 3 scales at home that do ranges of a few pounds with marked ounces to a heavier duty one that goes up to 30 lbs. You can often buy inexpensive postal scales off ebay though. As we do not live in the US, we do the labels at home, tape them to the boxes, and I drop them off at the post office on the side of the counter, skipping the line entirely. Apparently you can schedule a pick up though to happen when your regular mail is delivered:
USPS - USPS - Pickup Options
There isn't a true cost savings on the postage of the items (save you don't have to drive to the post office and back), but you do get free delivery confirmation on the printed label so there's a bonus. Pay Pal always wants your tracking number when things go wrong so that's a big plus. We much prefer the service, and I have nothing bad to relate about it.
Hope that helps!