Every time there is one of these misunderstandings I explain why
this happened and ask how they would like it to be changed so that
it doesn't happen any more, but sadly I have never really received
any concrete suggestions even from the people it has happened to. I'm
pretty sick of this happening so I want to do something about it.

So, I shall ask all of you, how would you expect it to work?

a) As soon as a mandate is authorised, just charge all existing
   invoices immediately

   Very tempting. Very simple. I fear there will be at least one
   person that will claim they never expected that to happen, and a
   returned Direct Debit has caused them to incur an eleventy
   billion pound penalty charge from their bank, their mortgage
   payment got rejected, and now there are men outside in shiny
   leather jackets.

b) As soon as the mandate is authorised, if the customer has
   existing invoices that are unpaid, there is a very noticeable
   message on the screen like:

       You seem to have unpaid invoices:

       #41234 £107.88
       #41239   £1.92

       Pre-existing invoices won't be automatically submitted for
       payment by Direct Debit. You can <a href="…">pay them now</a>
       by a one-off Direct Debit or any of our other supported
       payment methods.

I like (b).

I am open to other ideas if you have any. I can't really think of
any.