Buyers are hardly likely to be aware of the problem, but should put items in their basket and then pay in one amount with combined postage.
It is suggested that with buyer’s agreement, seller cancels sales and then relist as a job lot of 5 items. Unfortunately this gets complicated with buyer having to wait for refund from first payment/s. and buying again. There is also the risk of buyer loosing interest.
While you do not have to do this, you can combine, using one Simple Delivery label but you will only have proof of delivery for one item, with potential for Not received claims.