Question:
A company wants to sell boxes of various sizes, prices, and types. The price of the materials inside the box, if sold individually (retail), is higher.
For example, they prepare a box for electronic devices and place items in it worth €100 or more, but they sell the box for only €20.
The important point is that the buyer does not know what is inside the box; meaning, they buy it and take whatever comes out.
On the website, they post examples of materials or photos, but not everything is shown because the content changes, and this is done for the element of surprise.
What is the Sharia position on this?

Answer according to Fiqh al-Mizan (The Jurisprudence of Balance):
The Sharia ruling is that the sale of the unknown (Majhul)—a major ignorance—and that which contains Gharar (uncertainty/deception)—meaning the possibility of existence or non-existence, or increase and decrease, and the like—is invalid and not permissible, unless the cause of that uncertainty and ignorance, which is the reason for the prohibition, is removed.

Explanation:

First: One of the conditions for the validity of a sale transaction in Sharia is that the sold item must be known to both parties, especially the buyer, with a knowledge that removes the ignorance which leads to dispute. This is achieved either by specific designation (seeing the item with one's own eyes and knowing its type and amount) or by a precise description known in custom, such that no effective ambiguity remains in the item.
If what is inside the box is unknown in type, amount, or description, and the buyer purchases it on the basis of "whatever comes out, comes out" as stated in the question, this is exactly what jurists call: Bai’ al-Gharar (The Sale of Uncertainty).

Second: The Prophet (Peace Be Upon Him) explicitly forbade this type of sale. It was reported by Abu Huraira (may Allah be pleased with him) that the Messenger of Allah (PBUH) "forbade the sale of Gharar" (Narrated by Muslim). Gharar, as defined by scholars, is that which has an unknown consequence, or a matter where there is hesitation between two things wherein ignorance and probability dominate.
Among the forms of Gharar that jurists have textually prohibited are: selling the fetus in the womb, selling birds in the sky, selling fish in the water (before catching them), and selling by mere touch or tossing... All of these share the commonality that the buyer does not know the reality of what they are taking. This box with unknown content falls under this category; in fact, in some forms, it may be even more severe because the buyer knows neither the type, the amount, nor the description.

Third: The Gharar and uncertainty in this transaction is not minor Gharar that Sharia might overlook; rather, it is exorbitant (major) Gharar. This is because the buyer enters into a deal not knowing what their share is, with the possibility of receiving something they cannot use, or something that, in their view, is not worth the money paid, or finding things they have no need for originally. All of this generates dispute and animosity, and Sharia came to close the doors of dispute and animosity, as evidenced by the verse: {O you who have believed, do not consume one another's wealth unjustly but only [in lawful] business by mutual consent}. Mutual consent cannot truly exist regarding something whose reality is unknown to rational people.

Fourth: This transaction bears a strong resemblance to gambling (Maysir). The buyer pays a fixed amount (€20) in exchange for a "chance" to win something worth a hundred or more or less, entering the matter without knowing: will they be a gainer or a loser in terms of value and benefit? This is the exact image of gambling: paying money in exchange for a probability of unknown gain or unknown loss. Allah Almighty says in the description of gambling: {Indeed, intoxicants, gambling... are defilement from the work of Satan}. The intent of Sharia here is to protect people's wealth from entering a cycle of "pure risk" that is based neither on knowledge nor on a clear contract.

Fifth: The seller's statement on the website that "the value of what is in the box is equal to one hundred Euros" does not remove the ignorance. The buyer is not buying a "numerical value," but rather specific goods and items; they may benefit from them or they may not. Furthermore, the mentioned value might be based on an old retail price, or based on the seller's estimation, or based on things the buyer cannot verify. Sharia is not satisfied with this amount of general speech; rather, it requires either seeing the item or a precise description that removes doubt.

Sixth: The permissible forms that are close to this category are for the seller to define the contents of the box with a precise definition. For example, saying: "This box consists of: Device X of such-and-such type, with a charger, and headphones of such-and-such type, with a cable, and the value of these things is usually one hundred Euros, and we are selling it at a discount for twenty." Here, the item is known, and the matter becomes a matter of discount or clearance sales, which is permissible and contains no issue. However, for the contents of the box to remain unknown, and for only general examples to be mentioned, leaves the Gharar as it is.

Seventh: Based on what has been mentioned above; the format stated in the question—that the buyer does not know what is inside the box and buys the box as a surprise—is not permissible in Sharia (it is Haram). This is because it is a type of sale with exorbitant Gharar, and indeed contains a resemblance to gambling. The Prophet (PBUH) forbade the sale of Gharar, and Allah Almighty prohibited consuming people's wealth unjustly and commanded that trade be based on clear mutual consent built upon knowledge, not upon risk-taking and gambling.