Why can't they either state how much it will cost to post said item on the page the item is on or factor it into the price?

It really annoys me when you put something in your basket, checkout and then they slap a (usually larger than required) carriage fee on. All it does is annoy me, make me close my browser window and find a different shop. Pillocks.

Any online shop owners around care to comment on why annoying your customers is a good idea?

probably because most people will carry on to complete the purchase, if their "advertised price" was higher than their competitors they would miss out on a sale.

IMHO

well we choose to put post costs up front on each item
we do it because i find it annoying when the post charge is excessive and not up front

one reason is some people don't care and just complete the purchase
most on line businesses will analyse where customers drop out on transactions

another is that for those that use price comparison sites it makes the item look cheaper