Question

Hello and blessings,

Not long ago we bought a large number of items from a store for a renovation (that is, toilets, bathtubs, ceramic tiles, etc.).
In this kind of business, it is customary to arrive at some final price (the total for all the items), and then ask the seller for a discount and reach an agreed price.
And that is indeed what happened: the seller set a price of X (the total for all the items), we negotiated a bit, and in the end we agreed on X-Y.
A few days later we got a phone call from the store saying that there had been a mistake, and that in the original calculation (which produced X) they forgot some toilet or something, and they want us to pay for it in addition to the X-Y that we already paid.
We argue that we agreed on all the merchandise for X-Y. They say: the seller made a mistake when entering the items—are you asking to get that item for free??
We argue: we do not care about the price of each individual item; we totaled all the merchandise we agreed on at X-Y, and we did not check how much each item cost.

Please teach us, Rabbi, who is in the right?
Do we need to pay the price of the missing item, or is the seller’s mistake the seller’s/store’s problem?
The extra amount they are demanding is an increase of about 3–4% over the price we agreed on.