Customer payment terms default at 30 days

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Hi,

I am trying to change the default payment terms on my customer invoices.

I do not offer credit terms, payment is due on receipt of invoice.

I can go into an individual customer record and change it as I am creating the invoice but I need to change it from default to 0 days so I don't have to remember to change every invoice as they are created.

I know it can be done as per the Sage help site : 

"Change the due date on invoices

    When you enter an invoice, by default, the due date is set to 30 days. If you need to, you can change this for either all of your contacts or for an individual contact."

    I have followed the instructions there but only have the option to change it every time I create an invoice.

    I am probably missing something simple here as I am sure many small business/self employed don't offer 30 days credit terms.

    Thanks for any help.

    Andrew.