Obligation to pay when the seller is at fault

Peace and blessings

We recently bought a lot of products from a store (before renovation. That is, toilets, bathtubs, ceramics, etc.).
In the genre, it is common to have some kind of final price (the sum of all products) and then ask for a discount from the seller and reach an agreed price.
And it did happen, the seller set a price of X (the sum of all the products), we bargained a bit, and in the end we agreed on XY.
A few days later we received a call from the store that there had been a mistake, and by mistake in the original scheme (which created X) they had forgotten some toilet or something, and they want us to pay for it in addition to the X-Y we had already paid.
We claim that we agreed on all the goods for XY. They say – the seller made a mistake in entering the items, are you asking for the item for free??
We claim – we are not interested in the price of each item, we confirmed all the goods we agreed on in X-Y, we did not check how much each item costs.

Our rabbi will teach us the law with whom
Do we have to pay the price of the missing item or is the seller’s mistake a problem for the seller/store?
The addition they demand is an increase of about 3-4% from the price we agreed upon.