Are you sure they are asking for invoice? This will be a new one on most Amazon sellers, as I don’t think this question has ever been asked before.

Have you declared your non Vat registration on Amazon and enabled the automatic invoice procedure.?

If you are VAT exempt then you can enable Amazon to automatically provide invoice/receipt (it is actually a receipt if not VAT registered).
Click on this link and you can activate it.

Sellers exempt from VAT registration in both the EU and the UK can declare their VAT exemption status and benefit from automated receipt generation.

I have registered for VAT exemption so Amazon should supply the invoice? I’m not sure why they’re asking me

Have you checked if the invoice has been uploaded you can see it on the orders page? If it has, perhaps they just don’t know where to find it

If you go to your order screen and click manage invoice, you will know if one is there, if it’s not, its very simple to create one

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Have you actually enabled the automatic receipt generation by Amazon ?
Amazon will only do it if you have declared yourself exempt via this link:

. Sellers exempt from VAT registration in both the EU and the UK can declare their VAT exemption status and benefit from automated receipt generation.

Go to your manage orders page and find the order in question.
Click on “manage invoice” next to the order, download the invoice from the pop-up that displays.
Send the invoice to the customer through an Amazon message.

Check the date on the order, we regularly get customers from 3+ years ago asking for invoices, Amazon will only generate them if the order was placed AFTER you signed up for their automated invoice/receipt service.