I've seen a charge amount of 0.01 for a free trial. I'm pretty sure it was just a pre-auth, i.e. they didn't follow up and actually complete the charge.
Was it against a bank (ie, debit) account? If so, it's a pre-note test, fairly standard in the industry to ensure a direct-debit or direct-credit can be assumed safe.
This unit tests both the debit (of e.g. $.01) and a credit of the same amount against the target. They can also vary the amount, then ask the recipient to report the amount posted (so you can prevent additional fraud that way - attacker may have your bank account/routing details but no login to your statement/transaction listing).
It was definitely processed through the MC network, and not via Direct Debit (the closest UK equivalent to pulling funds via ACH).
I know this because the vendor didnt have my bank account number, because Direct Debits are processed in batch (not instantly), and because the app associated with the account showed it as a card transaction.
This is presumably an actually hard problem despite the comments. A pre-auth allows the acquiring merchant to capture that authorization and charge you. Sometimes these can be for $1 or more, depending on how aggressive the service is. Pre-authorizations are subtracted from your available credit/balance.
If the card approves the pre-auth, they are (presumably, AFAIK) on the hook for that $1 if the merchant captures the pre-auth. If many (unpaying?) users start charging you $1...
I'd guess they allow small pre-auths (up to $3 say), and then keep a blacklist of merchants who actually capture the charge.
Merchants on the blacklist have all charges denied.
Any 'free trial' who actually captures a dollar from their clients will cause lots of chargebacks from their customers, so would go out of business quickly... (A chargeback costs a business ~$15 in fees)
