We require separate invoicing for separate customers. Even in MSP account that doesn't seem to be possible. One workaround could be to create separate accounts probably utizling a gmail feature to use "+" sign to create unlimited email addresses. In that case you can provide multiple email addresses, but utilizes the single email account. Otherwise using separate email addresses for few dozens of site24x7 accounts is not practical.

Therefore could you allow "+" sign in email address. (alternatively you can enable separate invoicing per customer in a MSP account as well)