Customer Accounts are optional. You can sign up during check-out. You will be asked to enter your email address and a password that you can use to access your account by clicking on the sign-in button at the top of the page.
A customer account allows you to save your orders but unfortunately it does not save your PayPal information. To pay with PayPal you will also need a PayPal account which is free.
The forms are saved by email address. This means that if you use different email addresses you end up with more than one account. They cannot be combined, but it is still a terrific way to keep track of the designs you bought.