It means: Pay right now. Today.
"Invoice," usually just called a "bill"--the letter that demands payment, says how much to pay, and says when you need to pay it.
"Due"--an obligation, a requirement, a duty, a debt
"Due for payment"--a requirement to pay
"Upon receipt." Receipt is form of the verb "to receive." Receipt means "When you receive it," "When you get it."
"Terms." The agreement. We do A, B, and C for you, in return you do D, E, and F for us.
By contrast, an invoice might say "Terms net 30." That means payment is due 30 days from now.
This invoice says "Our terms are NOT net 30. Our terms are payment upon receipt." You don't have a month to pay, you must pay as soon as you get the bill.