Is activation absolutely required before continuing the process?
If not, you might try something like this:
Checkout form is one page. At the top is two or three fields for registration: email address, password, maybe their name. Then below that is just their payment information. When they submit the form, it logs them in and takes them to their dashboard.
My favorite part: the fields at the top for registration are also used for login. If the user types a recognized username/email and password, it automatically logs them in and shows their credit cards on file (or whatever you do to make payments convenient). If their login is not recognized, treat it as a new user.
You can see something like this on SmartyStreets' checkout which I designed. (Note that you don't have to actually buy something or complete the checkout process to see how it works.)
Then leave the activation for later; whatever you do, don't delay their payment. If they want to pay, let them pay now. Take away the stumbling blocks. Activation can usually happen after-the-fact.