In the collection letter sequence the option to block a customer can be set up. This will block the customer for all (field "Invoicing and delivery on hold"). In that case a payment can't be posted to the customer. It would be nice to have the option so the field "Invoicing and delivery on hold" will be set to Invoice.

Since you end out collection letters to collect a payment it seems strange that you are not possible to post it because the customer has been blocked for all.