Issue with VAT invoices from Royal Mail

I was under the impression a VAT invoice had to have an actual invoice date, and a unique invoice number, and that they could not be changed (i.e. you have to do a credit note and new invoice to make a change). Royal Mail Click&Drop are issuing an "ongoing" invoice, e.g. invoice number 202503 (does not look like a unique invoice number) for "March 2025" (not an invoice date as such). I can download it and put it on my accounts. But if I buy more postage in March that invoice, with SAME invoice number, has changed to include the new postage! I don't seem to be able to email HMRC about this, so what is the best way to query this with HMRC. It seems very wrong to me and I assume needs fixing.  For now, I am making sure we only load invoices 1st of following month so we never have an invoice that disagrees with Royal Mail.

Hi RevK,
We can only state the requirements which need to be met in relation to VAT invoices and what information needs to be stated on them.
Please see the guidelines below:
16. VAT invoices
Thank you. 

Thanks, that seems to confirm that... 1. The invoice number must be unique - the ones from Royal Mail are not. 2. A date of issue is required - the ones from Royal Mail do not state a date of issue. 3. It seems to be clear that changing an invoice should mean issuing a new invoice or credit note - Royal Mail are constantly changing the same invoice. So, how do I advise HMRC of this serious error in invoicing procedure in contravention of VAT Notice 700 so that HMRC can talk to Royal Mail and get this corrected? I did post links to images of the invoice and changed invoice but you redacted that from my post.

Hi RevK,
You would need to reach out to Royal Mail regarding their invoicing arrangements.
You can only report third parties to HMRC if you believe that fraud is being committed.
See the details below:
Report tax fraud or avoidance to HMRC
Thank you.