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And there's an FAQ article: https://www.zen-cart.com/tutorials/index.php?article=229
In short, the answer is yes. Use the same credentials on each store. Zen Cart will automatically tell PayPal which store to send the transaction details back to.
As long as you have it working on one store, adding it to multiple stores is easy ... just set them all up the same as the one that's working.
And, lest you ask what to do with the IPN URL ... leave it set to whatever it's using today for a working store. Again, Zen Cart only needs it ON and VALID. It'll give the right details when it needs it.
Remember however, that your customers will see whatever name is on your PayPal account, even if it's not the same as your store name.
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