Getting postage right is the most important thing for internet sellers & it isn't easy. Buyers do want to know up front what cost is but for multiple purchases it is difficult to guestimate and don't want to waste time asking unless it is a special/cheap item. Where possible I offer free postage on letter items to encourage multiple purchases & it does work, buyers know they won't get slugged an extra charge so happy to buy more & for other items I refund overpaid postage so they'll come back knowing I won't rip them off on postage. It creates very good will. If you refund via paypal they refund you the fee they charged for the refund amount too if you do it properly.

It amazes me that those sellers who do not charge the correct rate for postage can be power sellers, many buyers mustn't check cost prior to bidding/buying and it does give eBay sellers a bad reputation, but it is a difficult thing to make postage charges compulsory due to the complexity of charges.

Best policy is buyer beware I suppose & always check sellers feedback.