Frontend: 1st page:

Login page:
fields:
Vendor Name
Vendor email

Compare if vendor email=vtiger email. If OK enters to the next page.
Next page have following fields:
PO number vendor email
PO amount in €
invoice number

Also this page has a link to FAQ with explanation how to submit invoices.

After pressing the button we compare if entered PO number exists in Vtiger and has status 'Approved'. If amount is same as EUR amount in Vtiger. If email is same as registered email. If these are OK then 'OK'. If one of the entered values is not the same as in Vtiger then 'Error'.

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2nd option (not needed)

Frontend: 1st page: -Fields to ask Vendor number + email. -Compare if Vendor number = vtiger vendor number and email=vtiger email. If OK enters to the next page.
2 Options. Either Show which invoices are not invoiced yet for this vendor. E.g.

  Date      Proj name         Product     Amount    PM

29 Jan 2019 Brieg_en_de_tr English to German translation € 4,80 PM_name checkbox
12 Feb 2019 Hoky_en_de_tr English to German translation € 2,10 PM_name checkbox
mark checkboxes and press button

After user presses the button we check if the invoice is not submitted yet (Status: Approved). Then change status to: Invoiced. Generate pdf invoice and add button “Download”. Automatically send confirmation to the user that invoice received. Also send invoice to invoices@ with the invoice and date.
Add copy of PO module? named 'invoices received from translators' and copy corresponding entries to this module. So that it has only submitted invoices (POs) with statuses 'Invoiced' or 'Paid'.