Seems to me the product supplier should have some right to a comment. It may be necessary to explain or remedy a situation.
Maybe something like debate rules: there are only three opportunities to speak. The customer makes a statement, supplier responds, customer responds, supplier, customer and finally, supplier.
Supplier would have the final word but it would be clear how the process worked, or not.
If a promised resolution was not fulfilled, mod could add a dated post until it was met.