Automatic subscription field check when manually paying invoice for that service

I think a good option would be to have the ability to block clients from over-paying by checking if they have a subscription ID in profile when they are manually paying an invoice. If the client has sub then it re-directs them to message to alert them of a automated billing agreement and to ask them if they instead want to "add funds" to their account for future payments.

I know whmcs technically does the same result but this way it would distinguish between the two and the client would have advanced knowledge that they was adding funds as credit and specifically not "over-paying".