One basic issue is that as a regulated industry, its rates, including a "no change" policy are subject to government review and have its blessing if approved.
As it happens, there is no longer a "no change" policy, because at the site of the tolling, the system is cashless, even though you can pay in cash on an invoice from a toll operator or by other means (which renders some of the analysis below moot).
Even if there were still "no change" tolls, while you might overpay once, often by $7.50, or $17.50, for not having change out of ignorance, after that your decision to drive on a toll road without having a pass or proper change is on you for not being prepared for an issue known to you. This loss is de minimus and wouldn't be worth litigating over even if the excess charge wasn't blessed by the tolling authority regulator.
Also, your one time "surprise" overpayment is far less than $7.50, or $17.50, because the extra charges associated with the pay-by-mail option usually aren't as large as that. And, in general, when you make a claim for damages in court, you have an obligation to show that you mitigated your damages by choosing the least damaging alternative. So, if there is, for example, a $3.99 pay near me fee added to the $2.50 toll, your damages are $3.99 even if you actually paid a $2.50 fine with a $10, $20, or $100 bill, because you could have limited your damages to $3.99 by electing the pay-near-me option.
Similarly, if you can be released by consenting to pay-near-me, or by paying $10, $20, or $100, you can't reasonably make a claim for extortion or entrapment for more than the dollar amount at which you could have obtained your liberty, because by not doing so, you have implicitly valued your liberty at less than that dollar amount.
The Coinage Act of 1965, 31 U.S.C. § 5103, which states that "US coins and currency are legal tender for all debts, public charges, taxes, and dues," doesn't help you either. The tolling authority accepts U.S. coins and currency. Nothing about the Coinage Act of 1965 requires a seller of goods or service to make change if you don't have the exact amount of U.S. coins and currency for the debt you need to pay. Indeed, you can pay a toll in California within five days in cash at a variety of locations, even if you don't pay it at the time (elsewhere in California it is 48 hours). The Coinage Act doesn't govern what the amount of the debt that you have to pay will be.
Indeed, in general, there is no federal law requiring companies to accept anything other than exact payment for debts owed to them by making change when paid in U.S. coins and currency. So, state law controls the issue, and the fact that the State of California's relevant regulators have approached the "no change" arrangement means that you have no state law remedy.