Unfortunately the solution is not very convenient, or many buyers are aware of it.
Buyers should put in their basket before buying in bulk, and postage combined.
Solution is long winded and with buyers agreement, is to cancel sales and relist as one item for buyer to buy again.
Main problem with that is that refund of original payment can prove embarrassing for seller who may not have the money released, and same for buyer who may not have money available to pay again. Refunds go back to source and if paid using a credit card refund can take up to 30 days.