a bit of a gripe of mine too

okay, the items are $29.99 free post, post = $13.40ea. for 600gm

you want five of them and don't want to pay $149.95 because you believe the post will only cost $16.70 because you sell similar stuff and know the postage for them.

You therefore inform the seller that you want to pay $100 for these five items because of the combined postage being cheaper.

To which the seller says they cannot do it, you then just inform them they have lost a sale and you move on.

Some people just don't want to make sales.