This will mean I have to put them in the same package and combine postage. So, my question is, having both items listed with 'simple delivery' will the buyer pay for each item's individual postage, or will it be combined automatically? Also, what if the combined postage exceeds weight / size restrictions of the original listing?

If the buyer pays for multiple Simple Delivery items in one go - i.e. one payment transaction, you will (very) likely receive one label which you use on one packet, sending those items together. The postage cost to the buyer is calculated by ebay and shown to the buyer at payment. It is likely to be less than the postage costs of all the items individually added together. The calculated postage cost may look to be too low (or possibly too high) but that is what ebay's AI have decided. The label...

Two auction items bought by the same person / username | Seller Central

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Two auction items bought by the same person / username | Seller Central | eBay UK