Hi,
I can assure you this is not an arbitrary limit; though it's not exactly my area of expertise.
As indicated in my earlier reply, WHMCS can override the value for this callback each time it initiates a transaction.
By 'callback', are you referring to the URL buyers are returned to and a parameter is appended to this URL, or are you referring to PayPal Instant Payment Notification (IPN)?
There, realistically, is no need to add the email address of each store unless WHMCS does a validation in the IPN callback which checks the recipient (the PayPal account email address) against the recipient it ought to be (the value for 'business' you entered in the WHMCS setup). But again, these values can be the same for each store.
In short,
I would suggest setting the 'business' of the 8th WHMCS installation to your primary PayPal email address, and simply testing the checkout. This should work.
... View more