I have searched and cannot find an obvious solution to my problem which is as follows -

In my store there will be items which require collection as well as items which can be sent by courier/post.

Each item available for courier/post has its own individual shipping cost (I use the weight mod from picaflor-azul.com to add the cost in the weight field).

The issue is that if more than one item is purchased it could result in a mix where there should be no shipping fee as one or more items are to be collected - so they could all be collected, or if multiple items with shipping are chosen the total shipping may be different due to economy of scale.

The way zen-cart works the customer has to purchase and pay for shipping fees they should not incur, and I then would have to refund the difference manually.

It would be better if customers could order items but not pay for them until I have an opportunity to amend the order prior to them paying, then for them to pay using Papal.

Does anyone have any ideas if/how I could do this?

(Alternatively, can anyone suggest alternative methods which removes the need for refunds.)