Hi flobadob, I sell a lot on eBay and I admit that unless they are "collect only" items, I always use RM - as joko says, it's simple for us to go to the Post Office to send, although I know we can use DHL from our stationers, I don't see the need. I charge a little extra for a box, brown paper, tape, and for me to get to the PO - for instance something today was a 99p purchase but the p&p on the sale was £2.50 - actual postage was £1.72 plus approx 50p for the packaging and petrol etc. I do refund postage if I have overcharged by a fair bit (and sometimes I've overcalculated, I do weigh everything and assume a box weight - cat food boxes are brilliant for packing!) and as I too object to paying over the top for P&P, it's not unknown for me to refund 75p-£1 if it's warranted.
There is no relationship between the cost of the item (which could be as little as 1p) and the cost of getting it to you. Royal Mail is - for general items - our cheapest and easiest option.