If you own a PayPal account you must log in using the login space provided on PayPal's payment page at checkout. If you don't have a PayPal account you don't need to create one. Just fill in the section provided for credit card info.
Why do some products show up in multiple categories?
This is to make it easier for shoppers to find what they're looking for. Because some products have multiple functions, different shoppers will look in different categories depending on their needs. So having a product in multiple categories will help avoid frustration.