Some great advice above.

Another example which I use myself as 90% of my customers make multiple purchases.

When free post is offered, (and I do include the full cost of postage and handling in the cost each individual item): I work out the actual cost of postage for all items being sent then deduct any over-paid postage and make a refund of that amount.

However, when each item is paid for with a separate paypal payment I do as above then reduce the refund by 30c for each payment, which is paypals transaction charge for each separate payment. I send a note to the buyer thanking them for their trade, then suggesting it may work out better if they await a combined invoice before paying.

If they are using an app on a mobile divice, I suggest they await payment until they can make it from a computer or use the "classic site" to make a combined payment.

I have not had any problems with this and I've done thousands of transactions that are for combined "free post" items.