Payment terms begin upon receipt of the invoice, not from the invoice date. If the invoice was received by regular mail, the time of receipt is always disputable. Therefore, just stay flexible; a 10-day discount is still fine even after 12 days. No one will fuss about 1-2 days, who knows what the postal service was up to. And if they do, you can apologize and then transfer the payment. I have met enough companies in my professional life that always take the discount, even if it is not agreed upon in the payment terms. It works. Exploiting payment terms up to the last day (why not) or taking the discount even though the deadline has been significantly(!) exceeded. In most cases, this goes unnoticed.