I did not find such information on the 2Checkout website.
I have got an answer from the 2Checkout support:
The minimum recommended charge is $1.00 as banks may elect to reject lesser payments. You must charge a positive value with 2CO, so no negative or zero-amount sales are permitted.
There is no upper limit to a payment amount.
The system will not return an error if the value charged is at least positive - we are unable to account for a bank’s decision to allow or reject a payment based on a very small amount charged.
We recommend that you charge the approximate equivalent of $1.00 USD, regardless of the currency used during the test.
It is implemented in my extension: