Some banks do charge for declined ACH transactions as though they are bounced check fees, but that's something you'd need to check on with your specific bank. If yours does, that might be a good reason to change banks.
You're more likely to incur big fees from whoever was trying to run the ACH debit on your account. There's almost always fine print in those agreements with merchants spelling out what happens in the event of an ACH decline.
Beware that your bank doesn't honor an ACH debit request even though you don't have the money now, because they're very likely to hit you with all of the crazy fees associated with overdraft protections. THAT can get very expensive very quickly. My ex-wife did that on one of her accounts, and a $10 fill-up at the gas station turned into $90 in fees.
The rule of thumb is to know before you set up an ACH agreement what the fees are if something happens to cause the ACH to not be honored, both in terms of what that merchant and your bank will charge for it. You may decide the potential penalties are not worth the convenience factor, but at least you won't be shocked when the bill hits of you do proceed.
I hope this helps.