A bit of a scam in my book. You open an account and pay the £10 minimum. You open an automatic DD top up, minimum £10. As soon as you cross first time, the crossing fee is deducted and you are charged another £10 top up. Their coffers are £18 odd in credit. Quite an amount if you don't cross regularly.
------------- cramming for my finals in the twilight zone
You don't have to open a DD, you can pay by credit card. This is the best method for drivers who cross a few times a year, use up the £10 & then top it up, certainly cheaper overall than paying £2.50 as you go.