THERE IS NO ACTION REQUIRED FOR MOST ZEN CART MERCHANTS
Zen Cart’s implementation of PayPal and Payflow capabilities does not rely on specific IP addresses. Instead, it leverages PayPal’s preferred approach of using DNS host names. Thus, no action is required to support their change of adding IP addresses.
CAVEAT: If your hosting company has added firewall rules to their infrastructure specific to Payflow IP addresses, then they will need to update their allowed IP addresses using the list supplied by PayPal. In modern practice it is rare to see such firewall rules used, but if your setup is using them, then your hosting company will need to follow the instructions PayPal gave you.