If your payment processor takes a per-transaction fee (not all do) then yes, this is a slightly worse deal for the customer. However, I think this would still be a good choice to give them, even if many would probably not chose it in order to save a few cents.
If I burn 1 € per day and my max owed based on whatever risk assessment is 20 €, I can set my payment treshold to 15 €, meaning if a payment fails, I have 5 days to fix it and settle the debt before you suspend my access. If the trigger amount is the same as the max owed, I have zero time (well, presumably there is already some wiggle room for the time it takes to process the transaction).
I see what you mean — yes, this could be useful to some customers. We already implement a small grace amount above the max-owed limit that allows for continued service; your idea would essentially allow the customer to increase the default grace amount.