What these have in common
They run in your browser and keep nothing. What you type is not sent to us and is not written to your browser's storage, so it is gone when you close the tab.
The generators write, and you send. Each one gives you text to copy, or opens it in your own email app. None of them sends anything on your behalf, and an email opened that way cannot carry an attachment, so attach the invoice yourself.
The checkers check form, not facts. A well-formed IBAN or VAT number can still belong to someone other than the person on the invoice. Each checker says what it can and cannot prove, and links to the official lookup where there is one.
None of this is legal or tax advice. Where a tool mentions a rule, it names the country and the source. Check your own contract and your own tax authority before you rely on it.