1. You basically just have to give PayPal one valid functioning URL in order to check the box to turn the feature on. Zen Cart then overrides that value with the correct store's URL when actually submitting transactions. The setting you enter by hand is used if you submit a transaction to your PayPal account that didn't come from a store where the override URL was supplied with the transaction.
Related article: https://www.zen-cart.com/tutorials/i...hp?article=229

2. It's HIGHLY recommended that you use Express Checkout instead of Website Payments Standard, as it's much more reliable.