tinkerbell, You can print and pay for the postage/address labels on ebay ,then you can just leave them on the counter at the post office. I see people doing this when I go to the post office. Also in our town there is a Royal Mail sorting office with a public counter, again you can print postage/ address on ebay and just leave them at the sorting office.
What you do sell by the way, I thought I was doing well on eBay but I rarely sell 18 items in 2 months never mind a day !