The official tax form generated on the Policía Nacional sede. The SAME 790-012 form is used for several different government fees — you tick a different 'concepto' (concept) depending on what you're paying for. In the TIE renewal flow you pay it TWICE: once for the renewal authorization (before submitting Mercurio) and again later for the physical TIE card (before the huellas appointment).
Generate the form online at https://sede.policia.gob.es/Tasa790_012/, fill in your personal details, pick the correct 'concepto' for what you're paying, print it, then pay at any participating bank (CaixaBank, BBVA, Santander, Sabadell). Some banks also accept payment via app or ATM by scanning the barcode on the form.
The 'concepto' you tick determines what fee you pay. For the TIE renewal authorization (paid before Mercurio): tick the renewal/prórroga concept. For the physical TIE card (paid before huellas after approval): tick 'TIE que documenta la primera concesión de la autorización' (or the renewal-card equivalent). Your name, NIE, and address must match the rest of your file exactly.
Forms occasionally get revised. Always download a fresh copy on the day you plan to use it.