> However, I couldn’t find help instructions on how to add this to the wppizza site.
assuming you have the plugin/gateway enabled of course, you will find the settings under wppizza->gateways (where you will also find a link regarding other info and troubleshooting if you have issues)
>Also, we don’t want to deliver everywhere. How can we limit address postcodes or distance from the restaurant address?
this will probably help (see NOTES section)
https://www.wp-pizza.com/downloads/wppizza-delivery-by-postcode/