A single pamphlet or a thin paperback book will still go as a large letter and the rates for those are unchanged even if they weigh over 100g.

There is certainly a problem with multiples costing a lot more than single items once there into packet size.

I just do a 50p discount for each item purchased after the first, I don't phrase it as a postage discount just as a discount for multiple purchases. I have a flat rate £1 postage for 95% of items just because that's easier to send a discounted invoice than if the items are free postage.

But I will certainly be sending some multiple item purchases as split packages if it saves on postage costs.