You need to look at your bookkeeping business as a "business" and not a "charity", sorry to sound a bit harsh. If you make your business more efficient, in business, you don't always have to pass the efficency savings to the customer.
It is admirable that you feel that you have to pass this saving on to the customer, but you don't awlays have to.
A business gets an item manufactured and it costs them say £70 and, they sell it for £100. Now, if the product really takes off and suddenly they need to ramp up production to meet demand and, they then negotiate volume discount from the manufacturer to say a cost of £50.00 per item, you can bet you bottom dollar that, that businesses (and pretty much all businesses) will continue to sell it for £100 and make the extra money. It's called business. Virtually, every business in the world does this, whether they are big or small.
That's why you need to go to fixed price bookkeeping. If you make a saving, it's your profit.
As they say "It's not personal, it's just business".
Appologies if this sounds ruthless, but in business it is never personal, just business.