So the two main calls of action to direct someone to paying, that come to mind for me are
- View Cart
- Checkout / Check Out
I guess you could many other variations like order now, pay here, etc. But for best conversions, its my belief to stick with what people are familiar with. Those are the most popular two I can think of.
So they are two different pages however. One is a page where you can view, edit, delete items in your cart, the other is where you enter your shipping and payment info.
The main call of action to start the payment process is usually in the top right corner. This is where people expect it to be, so its probably the best place for it. It will usually say how many items you have in your cart, and contain a call of action such as "view cart". But what if the visitor just wants to check out? Will they know they have to view cart first?
My process is this: I managed to have the edit/delete cart items on the same page as the shipping and payment page, i.e. just one page, one step, one process. As the top right area, contains items in the cart, I think View Cart is an apt label for the link to the next step.
However, these days when you add to the cart, it is done in ajax, so no page refresh, however you need some client side notification to let the visitor know the item has been added. So perhaps a popup, saying you have added the item to the cart. Then a link with the text "Check Out Now".