The only way combined postage works on similar sized items, on Simple Delivery,  is for the buyer to first place those items in their basket,  and then proceed to eBay checkout to pay.

I would suggest to leave some message on your listings for example......  ' I'm happy to combine postage,  for this to be possible,  please place all items into your basket first before proceeding to eBay checkout to pay. '

@ec_coleus_and_crafts