Contracts have to be negotiated and signed by two parties to be valid. You have to have an opportunity to modify the terms before exchanging money. Buying commodities doesn't meet that standard. Neither do post-purchase EULAs.
This is patently untrue, on many points.
First, there is generally no requirement that contracts be signed, or even in writing. A very few types of contracts are governed by the Statute of Frauds, which specifies that there must be a writing signed by the party against whom a term is being used. The specifics vary from jurisdiction to jurisdiction, but the linked Wikipedia article is reasonably representative. Outside the Statute of Frauds, there's nothing wrong with an unsigned, or even oral, contract.
Second, you do not have to have the opportunity to modify terms period, let alone before exchanging money. Terms may be offered on a "take it or leave it" basis; the Uniform Commercial Code, section 2-207 provides for how negotiation happens, and expressly includes an option to forbid any alternate terms. At common law, the principle is the same: there need not be an opportunity to make a counteroffer.
In this context, it would be entirely unreasonable for us to assume that respondents -- or any other cruise passenger -- would negotiate with petitioner the terms of a forum-selection clause in an ordinary commercial cruise ticket. Common sense dictates that a ticket of this kind will be a form contract the terms of which are not subject to negotiation, and that an individual purchasing the ticket will not have bargaining parity with the cruise line.
Carnival Cruise Lines, Inc. v. Shute, 499 U.S. 585 (1991)
As to "before exchanging money," it is very common for terms to be left open at the time of acceptance and tender:
Transactions in which the exchange of money precedes the communication of detailed terms are common. Consider the purchase of insurance. The buyer goes to an agent, who explains the essentials (amount of coverage, number of years) and remits the premium to the home office, which sends back a policy. On the district judge's understanding, the terms of the policy are irrelevant because the insured paid before receiving them. Yet the device of payment, often with a "binder" (so that the insurance takes effect immediately even though the home office reserves the right to withdraw coverage later), in advance of the policy, serves buyers' interests by accelerating effectiveness and reducing transactions costs. Or consider the purchase of an airline ticket. The traveler calls the carrier or an agent, is quoted a price, reserves a seat, pays, and gets a ticket, in that order. The ticket contains elaborate terms, which the traveler can reject by canceling the reservation.
ProCD, Inc. v. Zeidenberg, 86 F.3d 1447 (7th Cir. 1996)
The ProCD opinion also cited to Carnival.
I cite specifically to ProCD because it was a post-purchase EULA case, and it directly contradicts you.
In short, every single point you made is precisely and exactly wrong.