As a Private seller , as you are selling used unwanted items, you are using, should be posting for items which comply, using Simple Delivery.
Here as marco@ebay has explained if buyers first place items in their basket, postage is combined for them at checkout.
Perhaps on your listings you can add, as I know many peeps are still not sure how combined postage works when sellers use Simple Delivery, something like.... ' I am happy to combine postage, but to facilitate this, please first place those items in your basket, and a combined postage will be shown for you at eBay checkout. '
@astrasri1