Please do not contact CS about this, as the seller is doing everything correctly, except for one thing, explaining their actions to you!
Under Simple Delivery rules, which Private sellers have to abide by, the only way to combine postage, is for the buyer to first place all items into their basket before paying. If they don't, they will be charged for all the subsequent postal charges showing per item.
The one way round this is for the seller cancel the items, relist them under One listing, so there is one postage, and one tracking number .
There are only 2 causes for cancelling items which do not incur a penalty, a defect for the seller's account and one is Problem with Buyer's Address. That is why they chose this option.
As said, their actions are correct to save you money on postage, but communication a little lacking in explaining their actions.
If the items have all been relisted as one, bid / but in confidence , the cancellation has fully refunded you, so you can buy and pay again.
Believe me, this is a pain for both sellers and buyers, the lack of clarity on combining postage.
@elverdundee