I've always received payment for which a proof of payment request is provided.
I've had one or two instances where I didn't receive a payment for which a proof of payment request was provided.
I've had three to five instances where I didn't receive a payment for which a proof of payment request was provided.
I've had six to ten instances where I didn't receive a payment for which a proof of payment request was provided.
I've had more than ten instances where I didn't receive a payment for which a proof of payment request was provided.