You are responsible for the fulfillment of customer orders. You may do this in any manner you wish, whether it be by offering prints, high resolution image downloads, or via other digital means.
If you are shipping physical product, you can set up your Paypal account so that the appropriate shipping charges are added to purchases. Information on shipping configurations can be found here.
After a customer places an order, you will receive an email stating that you have received funds through your Paypal account. Payment status is explained on the Paypal website, however, you should ensure that payments have “cleared” prior to sending any images to your customer.