The prices at Mercadona online are the same that you can find in your physical supermarket.
Mercadona offers a pricing system that is unique and exclusive called “Siempre Precios Bajos - SPB” or "Always Low Prices": All the products, all the time, always at low prices. With SPB you save more on the total sum of your order.
You must bear in mind that for products sold by weight, the final price is adjusted to the amount served. In addition, for products with a variable weight served on trays, the price that was current at the time your order was prepared is always the price you will be charged. You are not charged the final sum until your order has been fully prepared.