I would very cross indeed to to pay postage and handling of $18 in expectation that I would be receiving a suitably packed garment, and instead receive a post service with actual costs of no more than $7.40 at most, and maybe give or take a dollar or two extra for label printing, paper etc -with no care taken with the garment being stuffed into a satchel.

It is post piracy, no ifs or buts from my perspective.  Belinda, as you are aware you have paid more than twice the cost of the actual service used - ouch !! 

Whilst some buyers take the "total price - goods+post " approach, others, including myself, don't. 

If a seller lists post and handling costs additional to the item cost, I expect to get an appropriate post and handling service for the $$$ I have paid.

IMO the seller walks a fine line in terms of breaching ebay post and handling policy as well as the circumventing ebay fees policy. I would report them,

10 bucks profit on post costs for every sale they make with no ebay fees taken, come-on, not good for this trading place at all. In line with ebay policy, a sellers profit margin needs to be rolled into the item price, not the post charge.

If no refund was forthcoming from the seller for the overpaid post with an apology for their :"oversight".  I personally would leave low DSRs for post and handling, and maybe even a neutral weighing up my satisfaction with the transaction overall.