Invoice of/ for this month.this month's invoice
Which is the best way to say it on an email.
Please, find enclosed May's invoice

Thanks

Question

Invoice of/ for this month.this month's invoice
Which is the best way to say it on an email.
Please, find enclosed May's invoice

Thanks

Answer

This month's invoice, May's invoice are OK, and slightly less formal.
Your invoice for this month, your invoice for May, or probably even more commonly 'your invoice for the period ended 31/5' are a little more formal.