I agree with the comments here that El Corte Inglés doesn´t normally ask for ID .. and their smaller offshoot company , Opencor ( open 365 days a year and normally open until at least 2am ) also doesn´t always ask for ID . I always pay by cash ( efectivo ) .
One point about using cash ..I would ask the bank to give you banknotes of maximun €100 . Many places will not except the €200 and €500 notes . Corte Inglés is the exception again . I once saw a Japanese couple buying a few small things and they handed over a €500 note. The cashier put it through her fraud detection machine ... and gave them their change without a supervisor being involved . I was astonished .