Sounds like an auto-generated invoice, created by a computerised accounting system.
All phases of selection are entered separately into the system and a £0.00 charge is entered against that particular item. However, the system still chucks out the invoice and if you have an automated mailing system or an accounts assistant who isn't paying particular attention the invoices get sent out.
I used to have a similar problem with my accounting software but I wasn't so advanced to have things automatically posted so I simply binned those invoices.
My favourite cheque was from the Social Security for £ 0.01. I should have framed it but just for the hell of it I paid it into my bank account.
I wouldn't worry but if you're concerned give them a call.
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