I've ordered from Automaton several times.
First you create an account/login.
Then you add your item to your cart.
Click on your card and go through the checkout process when you're done adding items and you'll receive 1 (or 2) e-mails confirming your order/purchase.
For preorder items, they won't ask you to pay, they'll just send you an e-mail confirming your order.
For in stock items they'll also ask that you pay for the figure, plus taxes and shipping.
If you don't want something to ship right away (whether it's in stock or preorder), because you plan on buying more things (like the remaining constructicons) send an e-mail to
info@automatontoys.com asking them to place your item in Order On Hold (OOH). They'll hold it for you until it's "full" or until you ask them to ship it.
Mario is the guy who runs the store, and he and his staff are very friendly, and also post on these boards frequently. Don't be afraid to ask them if you need help with something.