How much volume/bulkiness as well as weight in what you plan to carry? A combination of handlebar bag, carradice (or similar)-style large bag under/behind the saddle, maybe a frame bag inside the front triangle might carry everything, provide more flexibility and weigh & cost less vs. a trailer.

If you want to go the seatpost rack approach, I have one (I think it's a Performance / Nasbar knock-off of someone else's product) but have not used it for a while. I think Topeak makes a very nice one (or side rails that mount onto their beam rack system to mount small panniers). When I did us the seat post rack, the rack had a tendency to twist just enough on the post that I worried about hitting the rear wheel. Stopped trying to solve the problem when I bought a steel frame (Bianchi Castro Valley) as a dedicated commuter/light tourer with proper rack and fender fitings. I was thinking of putting the seat post rack on my old road bike (w a aluminum seat post) to use it as a fast commuter; decided I was OK with an backpack for up to an hour and like riding my newer road bike better. Don't know if any of these seatpost racks are available in Korea. I would recommend avoiding any models with a quick-release lever/attachment for the seat post mounting, go with something that uses proper bolts (smaller & lighter at the attachment, too).