There are two separate flows scenarios to consider:
1. Fulfilling orders through Zenfolio partner labs. In this case, there's no need for a client to create a PayPal account if he or she wants to pay with a credit card. The client can pay with the credit card directly through Zenfolio. The customer can also pay with PayPal, and in this case, the PayPal payment goes to Zenfolio, not to the photographer. If the client chooses PayPal, the PayPal page offers two options - create a PayPal account or login with an existing PayPal account. Note, that in this scenario, photographer's PayPal account is not being used.
2. Fulfilling custom orders. In this case, the PayPal payment is made directly to the photographer, if photographer wishes to accept PayPal. As long as the photographer as a business account, this page will be displayed to clients. In this case, photographer's PayPal account is used, and has nothing to do with photographer's shopping cart.
If you have any further questions, please write to support at zenfolio.com and reference this thread.