In the United States, ACH is subject to abuse, because of the way it is structured.
ACH transactions can be reversed up to 90 days after they appear on the statement. This protection is guaranteed by law to the consumer. Unfortunately, the society we live in will always have nefarious players, who will take advantage of this and do a transaction and then reverse it. For regular and same-day ACH, the onus (burden of proof) of proving the transaction was genuine and was executed in good faith lies with the merchant and not the consumer. Solution providers will require you to place money in a reserve, that would be used in the event of a chargeback. When a chargeback occurs, the money will immediately be taken out from your reserve and paid to the customer’s bank. They will hold on to the money until either the chargeback dispute is settled in favor of the customer or after investigation, the chargeback dispute is in your favor. Either way, the money needs to be provided to be held in escrow, and that money comes out from your reserve.
Pre-funding is simply your spending limit. Unless of course, you bring your own payout partner in the beneficiary country, you would need pre-funding. We did a small video that explains the basics of pre-funding, which you can view here: What is pre-funding and how does it work?
Please also be cognizant of the Minimum Commitment (under your plan).