Mercadona (supermarkets) for cheap and surprisingly good cosmetics and toiletries (own brands Deliplus and Dermik). Mascara maxi volumen, nail treatments, anti-aging serums, oils, etc.
Shoes - Spanish brands are much cheaper in Spain. Look for Pretty Ballerinas, Ursula Mascaró, Vialis, Chie Mihara, Pura López, Camper, Jaime Mascaró, Vialis, Rebeca Sanver...
If in Madrid, go to Calle Augusto Figueroa for outlets specialised in mid to high end shoe brands. This is in Chueca, at the back of Gran Vía, Hortaleza / Fuencarral area, so easy to find.
Clothes - there are plenty of multi-brand boutiques. In terms of clothes shopping Madrid is much better than Barcelona. Depending on what you are looking for, the best shops are, imho, around Calle Almirante, in the area between Chueca, Alonso Martínez, Recoletos and Gran Vía. Another interesting area is the 'callejones' in Calle Jorge Juan, to the south of the Salamanca district.
Handbags - if Loewe is expensive, try Salvador Bachiller or Piamonte (in Calle Piamonte) for inexpensive stylish leather goods.
If you go to Valencia, do not forget to drink horchata, tigernut milk. Simply delicious!