Jumping in for Max here. It was actually pretty tough for us to figure out how to name ACH as a payment method. One of our goals going into Mercury was to stay away from acronyms or confusing bank terminology as much as possible. Most banks hide ACH behind "bill pay" or use nonstandard terms like "electronic check" or "e-transfer" which are also confusing and don't describe the payment method clearly. We wanted to expose ACH directly but couldn't figure out how to get across what it was without confusing users further. Hours of debate in the office ensued.
Eventually we decided to just stick with the acronym after finding out pretty much everyone we talked to knew what ACH is.