You should get the option for Custom if your item is less than £10.
I was playing around with it yesterday. I made a listing for an item for £3, I had the Custom option, but if I changed the price to £12 and clicked out, a little note came up at the bottom of the screen, 'Postage options updated' and Custom had gone. Switched it back to £3, got the note and Custom was back!
It may have been a test, but if so, I hope they keep it as it worked well for low value items that can be sent with a stamp.
