Currently we have this feature available with our recurring invoices. If your November invoice is overpaid before the December invoice is created, the November overpayment will roll forward into the December invoice.
We have a request in our tracking system to be able to carry a balance per customer. We haven't written specifications for this feature yet, so we don't know if a credit would automatically apply to the next invoice created. This feature will require some significant redesign of our database and since we haven't received a lot of interest in it, it hasn't made it to the top of our feature list.
If anyone is reading this post and would like to see this feature, please add to this post. If we see a lot of interest before our next release, we may choose to add it. We haven't completed feature selection for our next version, but at this point customer credits / overpayments has not been included.
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Jeremy
DotNetInvoice Support